Building bare-bones AL2023 container images

To create a container with bare minimum dependencies for an application

1. Determine your runtime dependencies. This will vary based on your application.

2. Construct a Dockerfile / Containerfile that builds FROM scratch. The following example of a Dockerfile can be used to build a container that contains only the bash shell and its dependencies.

   
   FROM public.ecr.aws/amazonlinux/amazonlinux:2023 as build
   RUN mkdir /sysroot
   RUN dnf --releasever=$(rpm -q system-release --qf '%{VERSION}') \
     --installroot /sysroot \
     -y \
     --setopt=install_weak_deps=False \
     install bash
   
   FROM scratch
   COPY --from=build /sysroot /
   WORKDIR /
   ENTRYPOINT ["/bin/bash"]
   

   1. This Dockerfile works by:

      1. Starting a AL2023 container named build. This container will be used to bootstrap the barebones container, this container is not deployed itself, but generates the container to be deployed.

      2. Creating the /sysroot directory. This directory will be where the build container will install the dependencies needed for the barebones container. In a subsequent step, the /sysroot path will be packaged up to be the root directory of our barebones image.

         Using the --installroot option to dnf in this manner is how we create the other AL2023 images. It's a feature of dnf that allows installers and image creation tooling to work.

      3. Invoking dnf to install packages into /sysroot.

         The rpm -q system-release --qf '%{VERSION}' command queries (-q) the system-release package, setting the query format (--qf) to print out the version of the package being queried (the %{VERSION} variable is the rpm variable for the version of the RPM).

         By setting the --releasever argument of dnf to the version of system-release in the build container, this Dockerfile can be used to rebuild the barebones container whenever an updated container base image of Amazon Linux is released.

         It is possible to set the --releasever to any Amazon Linux 2023 version, such as 2023.5.20240624. Doing this would mean that the build container would run as the latest AL2023 version, but build the barebones container from 2023.5.20240624 regardless of what was the current AL2023 release.

         The --setopt=install_weak_deps=False configuration option tells dnf to only install dependencies that are required rather than recommended or suggested.

      4. Copying the installed system into the root of a blank (FROM scratch) container.

      5. Setting the ENTRYPOINT to be the desired binary, in this case /bin/bash.

3. Create an empty directory and add the content of the example in Step 2 to a file named Dockerfile.

   $ mkdir al2023-barebones-bash-example
   	$ cd al2023-barebones-bash-example
   	$ cat > Dockerfile <<EOF
   FROM public.ecr.aws/amazonlinux/amazonlinux:2023 as build
   RUN mkdir /sysroot
   RUN dnf --releasever=$(rpm -q system-release --qf '%{VERSION}') \
     --installroot /sysroot \
     -y \
     --setopt=install_weak_deps=False \
     install bash && dnf --installroot /sysroot clean all
   
   FROM scratch
   COPY --from=build /sysroot /
   WORKDIR /
   ENTRYPOINT ["/bin/bash"]
   EOF
   	

4. Build the container by running the following command.

   $ docker build -t al2023-barebones-bash-example

5. Run the container using the following command to see how minimal a bash-only container is.

   $ docker run -it --rm al2023-barebones-bash-example
   bash-5.2# rpm
   bash: rpm: command not found
   bash-5.2# du -sh /usr/
   bash: du: command not found
   bash-5.2# ls
   bash: ls: command not found
   bash-5.2# echo /bin/*
   /bin/alias /bin/bash /bin/bashbug /bin/bashbug-64 /bin/bg /bin/catchsegv /bin/cd /bin/command /bin/fc /bin/fg /bin/gencat /bin/getconf /bin/getent /bin/getopts /bin/hash /bin/iconv /bin/jobs /bin/ld.so /bin/ldd /bin/locale /bin/localedef /bin/pldd /bin/read /bin/sh /bin/sotruss /bin/sprof /bin/type /bin/tzselect /bin/ulimit /bin/umask /bin/unalias /bin/wait /bin/zdump
   

1. Create an empty directory and add the C source code and Dockerfile.

   $ mkdir al2023-barebones-c-hello-world-example
   $ cd al2023-barebones-c-hello-world-example
   $ cat > hello-world.c <<EOF
   #include <stdio.h>
   int main(void)
   {
     printf("Hello World!\n");
     return 0;
   }
   EOF
   
   $ cat > Dockerfile <<EOF
   FROM public.ecr.aws/amazonlinux/amazonlinux:2023 as build
   COPY hello-world.c /
   RUN dnf -y install gcc
   RUN gcc -o hello-world hello-world.c
   RUN mkdir /sysroot
   RUN mv hello-world /sysroot/
   RUN dnf --releasever=$(rpm -q system-release --qf '%{VERSION}') \
     --installroot /sysroot \
     -y \
     --setopt=install_weak_deps=False \
     install glibc && dnf --installroot /sysroot clean all
   
   FROM scratch
   COPY --from=build /sysroot /
   WORKDIR /
   ENTRYPOINT ["/hello-world"]
   EOF
   
   

2. Build the container using the following command.

   $ docker build -t al2023-barebones-c-hello-world-example .

3. Run the container using the following command.

   $ docker run -it --rm al2023-barebones-c-hello-world-example
   Hello World!