fix: restore gcc, musl-dev, python3 and py3-yaml to the image

REGRESSION, mine. The committed Dockerfile had drifted from the image actually
in service: the running ci3 image contained these four packages, the Dockerfile
did not, and I rebuilt ci4 from the Dockerfile. The rollout therefore removed
tools zegit's CI depends on.

Proof the deployed ci3 had them: its Gitea runs got PAST `make credentials-check`
— which shells out to python3 — and failed later at cover-check. The ci4 run
fails earlier, at `credentials-coverage: need python3`.

What each is for, so the next rebuild does not drop them again:
  python3, py3-yaml  the repo's own gates are Python — credentials-coverage.sh,
                     validate-admission.sh and feature-lint.sh all shell out to
                     it, and credentials-coverage parses manifests with yaml
  gcc, musl-dev      cgo, which `go test -race` in the Makefile test target
                     requires: a C compiler plus libc headers (musl on Alpine)

Verified on the real base image rather than assumed — on
code.forgejo.org/forgejo/runner:12.11.1, `apk add git git-daemon gcc musl-dev
python3 py3-yaml` yields Python 3.12.14, a working `import yaml`, gcc 15.2.0,
and git-http-backend on the git exec path.

The deeper lesson is that a hand-built image can drift from its Dockerfile
silently, and nothing notices until someone rebuilds. Publishing via CI (#2)
is what stops that recurring, since every future image comes from the
committed file.

Dockerfile

@@ -33,6 +33,12 @@ USER root
 # curl,tar -> setup-go / gitleaks download + extract their own arch-specific binaries
 # make     -> zegit Makefile targets
 # ca-certificates -> trusted roots for HTTPS to gitea/github/goproxy
+# gcc,musl-dev -> C toolchain for cgo; `go test -race` (the Makefile test
+#             target) requires cgo, which needs a C compiler + libc headers
+#             (musl on Alpine)
+# python3,py3-yaml -> the repo's own gates are Python: credentials-coverage.sh,
+#             validate-admission.sh and feature-lint.sh all shell out to
+#             python3, and credentials-coverage parses manifests with yaml
 #
 # Go is intentionally NOT baked in: actions/setup-go fetches the pinned version
 # per-run (node + tar are present), so the image needs no rebuild on Go bumps.
@@ -45,7 +51,11 @@ RUN apk add --no-cache \
       curl \
       tar \
       make \
-      ca-certificates
+      ca-certificates \
+      gcc \
+      musl-dev \
+      python3 \
+      py3-yaml
 
 # Match the runner pod's runAsUser (1000); keep the image non-root by default.
 USER 1000