Docker | Build Docker images (docker build)
  1. Notes
  2. Build an image (docker build)
  3. Build cache
  4. Multi-stage builds
  5. Debug a failing build
  6. docker build option: --no-cache
  7. docker build option: -f FILE_NAME|FILE_LOCATION
  8. docker build option: --build-arg
  1. Notes
    Please see these pages for more information:
    Docker CLI: docker build (build an image from a Dockerfile)
    https://docs.docker.com/engine/reference/commandline/build/
    https://docs.docker.com/develop/develop-images/multistage-build/
  2. Build an image (docker build)
    The 'docker build' command has the following syntax:

    Let's use this Dockerfile:

    To build an image from a Dockerfile:

    The option '-t IMAGE_NAME:IMAGE_TAG' instruct Docker to create an image with the specified name (ubuntu-nginx) and tag (20.04).

    The character '.' specifies the current directory and it represents the path where Docker will look for any files and directories handled in the Dockerfile.

    By default, Docker will look for a Dockerfile with the name 'Dockerfile' in the current directory.

    You can add a ".dockerignore" file where you can list files and directories that you want to be excluded when building the image.

    The build command produces the following output:

    In this case it's the first time the image is built. All the steps are executed and no cache was used by Docker.

    To see the created image:
  3. Build cache
    Steps in the Dockerfile that produces image layers are cached and can be used by Docker when rebuilding the image which can reduces significantly the build time.

    Let's execute again the build command. The output should be similar to the following:

    As you can see, Docker verify for each build step if there is a cached layer, and if it does exists, it will use it (see ---> Using cache).

    Let's change the Docker file a bit (I will change the group id and user id):

    Let's execute the build command. The output should be similar to the following:

    As you can notice, the instruction of the step 2 was changed and hence Docker is building a new image layer. Docker will then execute the subsequent steps and won't use the local cache even if the remaining steps didn't change at all.
  4. Multi-stage builds
    Multi-stage builds feature allow you to add multiple FROM instructions in the Dockerfile. The FROM instructions are basicaly the same as the regular single FROM instructions that we have used previously. Each FROM instruction define a build stage that has its base image and a set of instructions to build images layers. A FROM instruction should have a name which can be used by other FROM instructions as a reference. The FROM instruction that references another one, will be able to copy artifacts created within that staging build.

    A staging build can be considered as a temporary build that we can use in the next staging builds to copy only the artifacts that we need and discards everything else. The goal is to have an optimized final stage (in term of disk space) that in theory will be the one that we will use for the image we want to create.

    Let's use this Dokerfile (not a very useful one but it's simple enough to demonstrate the multi-stage builds feature):

    Let's build the Dockerfile:

    Let's have a look at the image history:

    As you can see, the image does contain only the layers of the base image (alpine) and the layer of the COPY instruction.

    Notes:
    • Assigning a name to a stage build is recommended but not mandatory. Instead of using the name of the stage build you can reference it by its number. The first FROM instruction in the Dockerfile has the number 0 and the following will be numbered 1,2,3, and so on. To reference the first build stage, you do: COPY --from=0 ...

    • It's also possible to copy artifacts from external images (kind of using the image as a stage):

    • You can also use a stage build as a base image for the FROM instruction:
  5. Debug a failing build
    Let's use this Dockerfile above (notice the typo with the useradd command):

    Let's build the Dockerfile:

    In this case we can see the build complains about the command 'userad'.

    In some cases the error might not be very clear from the build output, so a nice feature in Docker is that you can use an intermediate layer of the image to start a container and debug any failing command.

    For example, we can run a container from the created layer '1e4467b07108' (see above) and execute commands directly in the container:
  6. docker build option: --no-cache
    You can use the option '--no-cache' to instruct Docker to not use cache when building the image:

    Let's use this Dockerfile:

    The first time we build the Dockerfile we will get the following output:

    If we build again the Dockerfile we will get the following output:

    You can notice that the step 2 has the mention '---> Using cache' which means that Docker just used the already built layer from this step.

    Using local cache can speed up build time; especially when avoiding rebuilding image layers that take long time to build or take up large amounts of disk space.

    In some cases (either to play safe or because we encountered some issues with the build) we might want to disable the cache, for that we use the option --no-cache:


    You can notice that the step 2 has the mention '---> Running' which means that Docker is rebuilding again the image layer for that step.
  7. docker build option: -f FILE_NAME|FILE_LOCATION
    By default Docker will use the Dockerfile in the current directory ('./Dockerfile'). You can use the option '-f FILE_NAME|FILE_LOCATION' to let Docker use a Dockerfile with a different name and/or a different path.

    Let's create a Dockerfile named 'Dockerfile1':

    To use the Dockerfile 'Dockerfile1' (located in the current directory):

    You can also provide the full path where Dockerfile1 is located (e.g. '/opt/dockerfiles/'):
  8. docker build option: --build-arg
    You can use the option '--build-arg' to set build arguments.

    Let's use this Dokerfile:

    Note that the arguments are used as variables in the Dockerfile using the syntax '$ARG_NAME'

    If you don't specify a value for an argument in the build command, then the default values in the Dockerfile will be used:

    To pass new values for the arguments, use the option '--build-arg':

    Check that the arguments were used as expected:
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