I would expect commands to run separately one after another: That won't be possible; a container runs as long as it's primary process is running; the container will stop the moment the first command completes. These base images then can be used to create new containers. The Alpine image does not have git, vim and curl by default, as you can see in the video. To prevent this, just make sure you use a specific tag of an image example: alpine:3. Externalize services by running them in separate Docker containers, for example. It can't be done from Dockerfile because database server is not launched at the moment and event it's hostname from docker-compose. However some developers, especially newbies, still get confused when looking at the instructions that are available for use in a Dockerfile, because there are a few that may initially appear to be redundant or, at least, have significant overlap. Understand build context When you issue a docker build command, the current working directory is called the build context. Dockerfile , in the order you would like them performed i. This tutorial will teach you how to define, build and run your own images.
This is a pretty lengthy article over 5500 wordsyou can think of it as a chapter of a Docker book. A Dockerfile is a text file that defines a Docker image. Recap of Docker base terms Let me repeat a few basic concepts to better explain. If you are absolutely new to docker, please start with the. You usually run one main process in one Docker container. You can think of this like one Docker container provides one service in your project. You can start containers to run all the tech you can think of, you can run databases, web servers, web frameworks, test servers, execute big data scripts, work on shell scripts, etc. A Docker image is a pre-built environment for a certain technology or service. The main source of Docker images online is the. You just need to search for your preferred tech component, pull the image from the store with the docker pull command and you are ready to start up containers. Containers are started from images with the docker run command. These layers contain the files and configuration needed by your environment. As you start up a container with docker run, Docker will add another layer on top of your image. While your image layers are read-only, the additional layer added by the container is read-write. This will actually happen most of the time, which means that learning about the Dockerfile is a pretty essential part of working with Docker. The Dockerfile contains a list of instructions that Docker will execute when you issue the docker build command. A Docker image is created by building a Dockerfile with the docker build command. This means that technology vendors and developers usually provide one or more Dockerfile s with their specific technologies. They define the steps of building dockerfile multiple cmd image in the Dockerfile and they use docker build to create the Docker image. All you need to do is to create a text file named Dockerfile with no extension and define your image. This tutorial will teach you how to define, build and run your own images. I attached the below Dockerfile from GitHub, this file is part of the. You can access the file directly via this url:. This also implies that understanding Dockerfile instructions is not enough to create your Dockerfile, because you need to also understand the context of the technology you are building for. The good news is that you can save a lot of time when starting out experimenting with a new technology, because you can use an image prepared by someone else, without understanding the details immediately. Reading Dockerfiles prepared by others is a great way to learn about technology. Use the command docker dockerfile multiple cmd in your terminal to list the images you currently have on your computer. Remember that images are stored on your computer once you pull them from a registry like the Docker store, or once you build them on your computer. If you have not pulled any images yet, your list may be empty. I pulled most of them from dockerfile multiple cmd Docker store, and I have built my own, too. It is worthwhile to check the image sizes in the picture. Please execute the following in terminal: 1. Create the Dockerfile Create an empty directory for this task and create an empty file in that directory with the name Dockerfile. You can do this easily by issuing the command touch Dockerfile in your empty directory. Congratulations, you just created your first Dockerfile. The Alpine image does not have git, vim and curl by default, as you can see in the video. This will be your first custom Docker image. The idea behind is that you need a starting point to build your image. I start my images mostly from other images. You can start you Docker images from any valid image that you pull from public registries. The image you start from is called the base image. You need to specify the directory where docker build should be looking for a Dockerfile. You should see a similar output in terminal now: 5. Enjoy the results Docker created an image from your Dockerfile. You should see a new image in your image list issuing docker images again. You should be seeing the version of vim and curl in your terminal. dockerfile multiple cmd We have successfully added two packages to the Alpine base image. At the headline of each step you can see the corresponding line in your Dockerfile. This is because docker build executes the lines in the Dockerfile one at a time. What is more important that with every step in the build process Docker will create an intermediary image for the specific step. This means that Docker will take the base image alpine:3. This means that the final Docker image consist of 4 layers and the dockerfile multiple cmd layers are also available on your system as standalone images. This is useful because Docker will use the intermediary images as image cache, which means your future builds will be much faster for those Dockerfile steps that you do not modify. Please issue the command docker images -a in terminal. You should see something like this: We used -a to list all images on your computer including intermediary images. Please note how the image ids are the same as the ones you see during the build process. The main advantage of image layering lies in image caching. This behavior makes our lives a lot easier. Since image layers are built on top of each other Docker will use images cache during the build process up to the line where the first change occurs in your Dockerfile. Every later step will be re-built. Please note that each layer only stores the differences compared to the underlying layer. The video may be misleading from this perspective, because I interperet the sizes in docker images -a differently. The right interpretation is that docker images and docker images -a display the size of the image including the dockerfile multiple cmd of parent images. Which means that the steps to install curl, vim and git will be run from scratch, no caching will be available beyond the point where the change occured. Our newly built image is ready to use, but the previous image that we built with curl is still hanging around and it does not have a proper tag or name right now. You can check the image ids to see that this is the same image we built previously. Docker calls such images dangling images. Dockerfile best practices Minimize the number of steps in the Dockerfile Minimizing the number of steps in your image may improve build and pull performance. I should write a file like this instead, where I order packages in alphabetical order. This is very useful when you work with a long list. Anyway, your image will stabilize after a while and changes will be less likely. Clean up your Dockerfile Always review your steps in the Dockerfile and only keep the minimum set of steps that are needed by your application. Docker will send all of the files and directories in your build directory to the Docker daemon as part of the build context. You can remedy this situation by adding a. You can specify the list of folders and files that should be ignored in the build context. If you want to have a look at the size of your build context, just check out the first line of your docker build output. My alpine build output for example says: Sending build context to Docker daemon 2. Which means that you should create Dockerfiles that define stateless images. Any state, should be kept outside of your containers. One container should have one concern Think of containers as entities that take responsibility for one aspect of your project. So design your application in a way that your web server, database, in-memory cache and other components have their own dedicated containers. Dockerfile key instructions best practices The official Docker documentation is usually very easy to follow and easy to understand. This will set the base image for your Dockerfile, which means that subsequent instructions will be applied to this base image. You will want to use this feature, for example, when you use one base image to build your app and another base image to run it. So in subsequent instructions the environment variable will be available. This is important because of layer caching. Having these on two separate lines would mean that if you add a new package to your install list, the layer with apt-get update will not be invalidated in the layer cache and you might end up in a mess. So let me put it in plain English. This implies that stuff stored in the volume will persist and be available also after you destroy the container. In other words it is best practice to crate a volume for your data files, database files, or any file or directory that your users will change when they use your application. The data stored in the volume will remain on the host machine even if you stop the container and remove the container with docker rm. The volume will be removed on exit if you start the container with docker run --rm, though. You can also share these volumes between containers with docker run --volumes-from. You can inspect your volumes with the docker volume ls and docker volume inspect commands. You can also have a look inside your volumes by navigating to Docker volumes in your file system. You can find out the id of the dockerfile multiple cmd and thus the volume by running docker inspect on your container. Now you may think that docker run -v. If the directory does not exists, Docker will create it for you. When you specify an entry point, your image will work a bit differently. How could you even do that, when these things will be available only later. So you can do something like this: Now that we looked at the toolset and best practices, you dockerfile multiple cmd be wondering, what is the best way of building your Dockerfile. Well, I think everybody has their own ways, let me show you mine. Writing the Dockerfile is fairly simple. The hard part is to know what steps you need to take to set up your environment. I use a fairly straightforward 4 step approach to build my Dockerfiles in an iterative manner. I usually check out different flavors, like an image based on Debian Jessie and another on based on Alpine. I also check out the images made by others for a specific technology. When working with php, I usually start from php dockerfile multiple cmd the Apache web server included and add my stuff myself. I pull my chosen images to my computer and start a container in interactive mode with a shell. I start manually executing the steps in the container and see how things work out. If something goes wrong, I change the course, and I update the Dockerfile immediately. Every now and then I stop and build my image from the Dockerfile to make sure that it produces the same results every time. Then I use the newly built image to start a container with a shell and go on with my installation and set-up steps. Please note that you can do a lot of fancy stuff for production applications and team work, like multi-stage dockerfile multiple cmd, image hierarchies, shared volumes, networked containers, swarm and a lot more.
