Docker final stage bakes BUILD_TIMESTAMP into /app/BUILD_TIMESTAMP via
build-arg (only consumed after mix compile, so earlier layer caching is
preserved). Application.build_timestamp/0 reads the file, falls back to
DEPLOY_TIMESTAMP env, then to compile time. Navbar renders a compact
"Deployed Xm ago" with the full UTC time in the tooltip.
setup-buildx-action downloads from GitHub Releases which is slow
and unreliable from the Forgejo runner. Plain docker build/push
works fine since Docker is already on the runner host.
