DockerHub's anonymous pull rate limit applies per IP for hours,
so retrying cannot ride it out on shared runners;
`mirror.gcr.io` serves the same official images without that limit,
as the io.js source-install job already does.
The full suite cannot run on Alpine: the install-based suites pin ancient Node (0.10.x, io.js) that has no musl binary and cannot source-compile on musl.
Run the fast unit suite instead (mirroring the ubuntu runner: non-root via su-exec, passwordless sudo, a PTY, no system node), plus a binary-only regression matrix that installs every (Alpine, Node) pair with a real unofficial musl binary via `nvm install -b`: x64 back to node 8.17.0 on old Alpine, arm64 at the v20.20.1/v22.21.1/v24.9.0 floors on modern Alpine.
