Fold build-tools into this repo

build-tools is the image the pipeline's build pod runs its shell steps in.
It was owned by devops-lib, which is an odd home: it is not library code,
it is an artefact built by hand and pushed to Harbor, exactly like the
mirrors here. devops-lib only ever referenced the result by tag, and it
still will.

Three deliberate changes from the homelab's version:

- Pushed to base-images/build-tools:1, not homelab/. That project is
  public, so build pods pull it with no credentials — the same reason the
  language images live there.

- FROM the mirrored docker:27-cli rather than Docker Hub, with that tag
  added to images.txt. Otherwise building the image that exists to remove
  a Docker Hub dependency would itself depend on Docker Hub.

- yq is pinned instead of "releases/latest". An image that resolves a
  different yq on every build is not reproducible, and that is the kind
  of drift that surfaces months later as an unexplained pipeline failure.

The README gains the build-and-push procedure: the same pod shape as the
mirror, with the registry CA mounted into dind so the push is trusted, and
DOCKER_BUILDKIT=0, since BuildKit wants to write state under /root/.docker
where the push credentials get mounted read-only.

Verified the three places that must agree do: images.txt, the README's
inlined ConfigMap copy of it, and the Dockerfile's FROM tag.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LEsTefWWifp4ikvhHF5s6N

build-tools.Dockerfile

@@ -0,0 +1,37 @@
+# The docker-cli image the pipeline's build pod runs its shell steps in.
+#
+# Bakes in everything the stages need — git, yq, bash, python3 with pip and
+# venv for runHooks' python hooks, curl — so nothing is installed on demand
+# on every single build. Installing tools per build was slow and, worse,
+# quietly undermined reproducibility: a build's behaviour depended on
+# whatever the package mirror served that morning.
+#
+# Lives here rather than in devops-lib because it is the same kind of thing
+# as everything else in this repo: an image built by hand, occasionally,
+# and pushed into Harbor for builds to pull. devops-lib only references the
+# result by tag.
+#
+# Built and pushed manually — never by a Jenkins job, which would need this
+# image to already exist in order to run. See README.md.
+#
+# Pushed to base-images, not homelab: that project is public, so build pods
+# pull this with no credentials at all, which is the same reason the
+# mirrored language images live there.
+#
+# The tag is pinned explicitly by devops-lib-gcp's dind-pod.yaml, so
+# rebuilding this does not roll anything out until that pin is bumped too.
+# Bump the tag when this file changes; do not overwrite an existing tag.
+FROM harbor.35.238.248.203.nip.io/base-images/docker:27-cli
+
+# Pinned rather than the homelab's "releases/latest": an image that resolves
+# a different yq every time it is built is not reproducible, and this is
+# exactly the sort of thing that changes under you months later. Bump it
+# deliberately.
+ARG YQ_VERSION=v4.44.3
+
+RUN apk add --no-cache git bash python3 py3-pip py3-virtualenv curl \
+    && curl -sL -o /usr/local/bin/yq \
+        "https://github.com/mikefarah/yq/releases/download/${YQ_VERSION}/yq_linux_amd64" \
+    && chmod +x /usr/local/bin/yq \
+    && yq --version \
+    && git --version