ci: ARM64 container builds fail when Ubuntu ports mirror is unavailable

[thepastaclaw]

ARM64 container build mirror timeout

The ARM64 variants of both container-build jobs can fail before Dash compilation
when the Ubuntu ARM package mirror is temporarily unreachable.

Observed in Actions run 29439519396:

• Build container / Build container (arm64)
• Build slim container / Build container (arm64)

The corresponding AMD64 jobs passed.

Failure mode

During the Docker image's apt-get update, all
http://ports.ubuntu.com/ubuntu-ports Noble repositories timed out. Apt then
used incomplete/stale indexes and failed to resolve llvm-19-dev dependencies
such as libffi-dev and libxml2-dev, exiting with status 100.

This occurs before any PR source is compiled. The triggering PR changed only
governance source and tests, not the CI Dockerfiles or package setup.

Reproduction

Run either ARM64 container-build job while ports.ubuntu.com:80 is unavailable
or timing out. The Docker build reaches the LLVM installation layer,
apt-get update warns that the Ubuntu ARM indexes could not be fetched, and
apt-get install llvm-19-dev subsequently reports unresolvable dependencies.

Possible hardening

Consider using HTTPS for the Ubuntu mirror and/or configuring apt acquisition
retries so a transient mirror/network outage does not fail the entire CI matrix
before compilation.