Docker Multi-Arch Images#

The cours-devops-docker course uses Docker images in lab exercises that must run on both amd64 (standard x86-64 PCs) and arm64 (Apple M-series, Raspberry Pi, Radxa Zero) machines. This creates a practical constraint: not all images referenced in the course support multiple architectures, and the workaround for university network access (a Squid proxy) compounds this because the most commonly cited proxy image (sameersbn/squid) is amd64-only.

Architecture Context#

Docker images encode their target architecture in their metadata. You can inspect it with:

docker image inspect <image> --format '{{.Architecture}}'

The course illustrates this in a debugging exercise where docker image inspect bad-nginx:1 reveals "Architecture": "amd64" — and the instructor notes this image won't run on Apple M1 machines 1.

Docker's buildx plugin enables building images for architectures beyond the host's native platform (including RISC-V). The course introduction references this capability in the context of running Docker on ARM devices such as Raspberry Pi and Radxa Zero 2.

Proxy Image: Multi-Arch Availability#

Course labs run behind a university network that rate-limits Docker Hub pulls by shared IP 3. A Squid proxy is used to work around this. Two proxy images are referenced:

Implication: The sameersbn/squid:3.5.27-2 command shown in course notes 6 will fail silently or error on arm64 hosts. Use ubuntu/squid instead when running on ARM hardware.

Proxy Setup in Lab Exercises#

The CMD vs ENTRYPOINT exercise uses Squid as a supporting proxy container. The minimal squid.conf used is the same in both the lecture notes and the code samples 7 8:

http_port 3128
acl all src all
http_access allow all

To launch the proxy with the multi-arch-compatible image:

docker run --name squid -d --restart=always --publish 3128:3128 \
  -v $(pwd)/squid.conf:/etc/squid/squid.conf ubuntu/squid

The course notes show the equivalent using sameersbn/squid:3.5.27-2 6; swap the image name for arm64 hosts.

CMD vs ENTRYPOINT: Lab Dockerfiles#

The primary multi-arch-safe images used in exercises are the alpine-based curl images. Alpine's official image is multi-arch.

• Base example (cmd-vs-entrypoint/Dockerfile): FROM alpine:3.24, uses CMD with a hardcoded proxy URL.
• my-curl:1.0 (my-curl/Dockerfile): FROM alpine:3.24, uses CMD ["curl", "-x", "http://prx:3128", ...] — targets the university proxy by default, with a commented-out variant for host.docker.internal when running a local Docker proxy.
• my-curl:2.0 (parametrable) (my-curl/parametrable/Dockerfile): Switches CMD to ENTRYPOINT ["curl", "-x", "http://prx:3128", ...], making the target URL an argument passed at docker run time.

University Proxy Configuration#

For students whose client Docker daemon needs to pull through the university cache (cache-ili.univ-artois.fr), the daemon-level mirror approach is the cleanest 9:

/etc/docker/daemon.json

{
  "registry-mirrors": ["http://cache-ili.univ-artois.fr:80/proxy_cache/"],
  "insecure-registries": ["cache-ili.univ-artois.fr:80"]
}

Then restart Docker: sudo systemctl restart docker. This makes all docker pull commands transparently route through the cache without changing image names.

For proxy settings inside Dockerfile builds, ENV is preferred over --build-arg when the proxy is always needed, because ENV persists across all subsequent RUN layers 10.

Key Source Files#