ci: carry the shared build-image script (infra#309)

Byte-identical copy of mathias/infra's ci-templates/build-image.sh. A
template that carries its own copy of the build LOGIC manufactures a new
copy of every bug in it -- which is exactly how rootless buildah reached
a dozen repos (infra#241). Now it manufactures a copy of one file that a
drift gate keeps current instead.

.gitea/build-image.sh

@@ -0,0 +1,108 @@
+#!/usr/bin/env bash
+# Build an OCI image with buildctl and push it to the local registry.
+#
+# CANONICAL COPY: mathias/infra ci-templates/build-image.sh (infra#309).
+# Every repo carries a byte-identical copy at .gitea/build-image.sh, written by
+# `task ci:sync` and held in place by the drift gate. DO NOT EDIT THE COPIES --
+# edit here, re-sync, and the gate will tell you which repos still disagree.
+#
+# Why this exists: this step was hand-copied into a dozen repos, so migrating it
+# off rootless buildah (infra#241) meant eleven near-identical commits, eleven
+# verifications, and three separate rediscoveries of the same fix in the two
+# repo templates and the gitea-ci skill. The differences between the copies were
+# incidental -- a label here, a build-arg there -- never intentional. Those are
+# inputs now, not forks of the implementation.
+#
+# Why buildctl and not buildah: the gitea-runner pod runs with
+# `capabilities: drop [ALL]` and `allowPrivilegeEscalation: false`, which
+# unconditionally blocks the setuid-root exec that rootless buildah's uid/gid
+# remapping needs, and the pod has no OCI runtime at all. buildctl is a thin
+# client -- it hands the build to the host's buildkitd over a socket, so the
+# privileged layer work happens on the host and this pod's securityContext is
+# not in the picture. buildah is absent from the runner image as of baked-v14
+# and its absence is asserted at pod start (infra#308).
+#
+# Inputs, all via environment:
+#
+#   IMAGE            (required) image name, e.g. cobalt-dingo
+#   REGISTRY         default localhost:5000
+#   SHA_TAG          default `git rev-parse --short HEAD`
+#   VERSION_TAG      optional extra tag, e.g. v1.2.3 on a tag build
+#   PUSH_LATEST      default 1; set 0 to skip the :latest tag
+#   CONTEXT          default . -- build context directory
+#   DOCKERFILE_DIR   default $CONTEXT -- directory holding the Dockerfile
+#   DOCKERFILE_NAME  optional, when the file is not named `Dockerfile`
+#   BUILD_ARGS       optional, space-separated K=V pairs
+#   EXTRA_LABELS     optional, space-separated K=V pairs
+#   GIT_SHA          optional, stamped as org.opencontainers.image.revision
+#   IMAGE_SOURCE     optional, stamped as org.opencontainers.image.source
+#
+# Note on shell style below: every conditional is a full `if` block, never
+# `[ -n "$X" ] && cmd`. Under `set -e` the && form exits the script whenever the
+# test is false, which is a real bug this estate has already shipped once.
+
+set -euo pipefail
+
+if [ -z "${IMAGE:-}" ]; then
+  echo "build-image: IMAGE is required" >&2
+  exit 2
+fi
+
+REGISTRY="${REGISTRY:-localhost:5000}"
+BUILDKIT_HOST="${BUILDKIT_HOST:-unix:///run/buildkit/buildkitd.sock}"
+CONTEXT="${CONTEXT:-.}"
+DOCKERFILE_DIR="${DOCKERFILE_DIR:-$CONTEXT}"
+PUSH_LATEST="${PUSH_LATEST:-1}"
+VERSION_TAG="${VERSION_TAG:-}"
+
+if [ -z "${SHA_TAG:-}" ]; then
+  SHA_TAG="$(git rev-parse --short HEAD)"
+fi
+
+REF="${REGISTRY}/${IMAGE}:${SHA_TAG}"
+
+TAR_FILE="$(mktemp)"
+trap 'rm -f "$TAR_FILE"' EXIT
+
+args=(
+  --frontend dockerfile.v0
+  --local "context=${CONTEXT}"
+  --local "dockerfile=${DOCKERFILE_DIR}"
+)
+
+if [ -n "${DOCKERFILE_NAME:-}" ]; then
+  args+=(--opt "filename=${DOCKERFILE_NAME}")
+fi
+if [ -n "${GIT_SHA:-}" ]; then
+  args+=(--opt "label:org.opencontainers.image.revision=${GIT_SHA}")
+fi
+if [ -n "${IMAGE_SOURCE:-}" ]; then
+  args+=(--opt "label:org.opencontainers.image.source=${IMAGE_SOURCE}")
+fi
+for kv in ${BUILD_ARGS:-}; do
+  args+=(--opt "build-arg:${kv}")
+done
+for kv in ${EXTRA_LABELS:-}; do
+  args+=(--opt "label:${kv}")
+done
+
+echo "build-image: building ${REF}"
+buildctl --addr "$BUILDKIT_HOST" build "${args[@]}" --output "type=oci,dest=${TAR_FILE}"
+
+# buildctl has no registry-push equivalent to `buildah push`, so skopeo copies
+# the OCI archive up -- once per tag, from the same archive, so every tag points
+# at bit-identical content.
+push_tag() {
+  echo "build-image: pushing ${1}"
+  skopeo copy --dest-tls-verify=false "oci-archive:${TAR_FILE}" "docker://${1}"
+}
+
+push_tag "$REF"
+if [ "$PUSH_LATEST" = "1" ]; then
+  push_tag "${REGISTRY}/${IMAGE}:latest"
+fi
+if [ -n "$VERSION_TAG" ]; then
+  push_tag "${REGISTRY}/${IMAGE}:${VERSION_TAG}"
+fi
+
+echo "✓ build-image: ${REF} pushed"