build: split the image into runtime and uat targets, publish multi-arch

.github/workflows/release.yml

@@ -79,10 +79,22 @@ jobs:
           registry: ghcr.io
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
+      # QEMU is what lets a single amd64 runner emit the arm64 half of the
+      # manifest list. Without it buildx silently produces amd64 only, which
+      # is how v0.1.0 shipped an amd64-only image while the release archives
+      # carried linux/arm64 (issue #5).
+      - uses: docker/setup-qemu-action@v3
+      # The `runtime` target, explicitly. Not the default-last-stage
+      # behaviour: naming it here means reordering the Dockerfile can never
+      # quietly publish the Ruby drill image to studios again.
       - uses: docker/build-push-action@v7
         with:
           context: .
+          target: runtime
+          platforms: linux/amd64,linux/arm64
           push: true
+          provenance: true
+          sbom: true
           build-args: VERSION=${{ github.ref_name }}
           tags: |
             ghcr.io/butterstack/butterstack-connector:${{ github.ref_name }}

Dockerfile

@@ -1,16 +1,29 @@
-# Multi-stage build for the butterstack-connector UAT/drill image
-# (connector.spec.js, issue #1574/#1575).
+# Multi-stage build with TWO publishable targets. Build with --target.
 #
-# The daemon itself is a static Go binary with no runtime dependencies. The
-# final stage is Ruby-based anyway because the UAT "studio LAN" stands the
-# real `p4` CLI up with test/support/fake_p4 -- a Ruby script bind-mounted at
-# /usr/local/bin/p4 by docker-compose.uat-connector.yml -- and the connector
-# execs whatever binary connector.yml names, argv-only, no shell.
+#   runtime  (default, and what `v*` tags publish)  distroless + the binary
+#   uat      (UAT/drill environment only)           adds Ruby for fake_p4
 #
-# NOT hardened for production (issue #1575 survival conditions 1/4/5: no
-# Sigstore keyless signing, no SBOM, no digest-pinned base image, no
-# reproducible-build docs -- see connector/README.md "what this does not
-# prove"). This image exists for the UAT drill environment only.
+# `runtime` is last on purpose, so a bare `docker build` with no --target
+# produces the safe artifact rather than the drill one.
+#
+# Why the split (issue #5): the daemon is a static CGO_ENABLED=0 Go binary
+# with no runtime dependencies, but the final stage used to be Ruby-based
+# unconditionally, which shipped a 33.9 MiB interpreter layer to every studio
+# that pulled a release. Worse, that stage's own header declared it "for the
+# UAT drill environment only" and "NOT hardened for production" -- and it was
+# exactly what v0.1.0 published and what studios ran.
+#
+# The Ruby is genuinely needed, just not here: the UAT "studio LAN" stands the
+# `p4` CLI up with test/support/fake_p4, a Ruby script bind-mounted at
+# /usr/local/bin/p4 by docker-compose.uat-connector.yml, and the connector
+# execs whatever binary connector.yml names, argv-only, no shell. So `uat`
+# keeps Ruby and `runtime` does not. The seven-drill harness is unaffected
+# either way: `make drills` runs `ruby test/drills.rb` on the HOST
+# (RUBY ?= ruby in the Makefile), never inside an image.
+#
+# Still unmet for production, tracked separately (issue #1575 survival
+# conditions 1/4/5): no Sigstore keyless signing, no SBOM, no digest-pinned
+# base images, no reproducible-build docs.
 
 FROM golang:1.25-alpine AS build
 WORKDIR /src
@@ -23,7 +36,10 @@ COPY . .
 ARG VERSION=dev
 RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w -X main.Version=${VERSION}" -o /out/butterstack-connector ./cmd/butterstack-connector
 
-FROM ruby:3.3-alpine
+# --- uat -------------------------------------------------------------------
+# Drill/UAT image. Ruby is load-bearing here and only here: fake_p4 is a Ruby
+# script the UAT compose file bind-mounts over /usr/local/bin/p4.
+FROM ruby:3.3-alpine AS uat
 
 RUN apk add --no-cache ca-certificates \
  && addgroup -g 10001 connector \
@@ -46,3 +62,59 @@ WORKDIR /home/connector
 # renders that file from UAT_CONNECTOR_* env vars first (see that script's
 # header for why that is a UAT-only pattern, not a protocol exception).
 ENTRYPOINT ["/usr/local/bin/butterstack-connector", "-config", "/etc/butterstack/connector.yml"]
+
+# --- skel ------------------------------------------------------------------
+# Builds the unprivileged account and the directory skeleton for the runtime
+# stage, which has no shell of its own to run adduser/mkdir in.
+FROM alpine:3.22 AS skel
+RUN addgroup -g 10001 connector \
+ && adduser -D -u 10001 -G connector -h /home/connector -s /sbin/nologin connector \
+ && mkdir -p /skel/etc/butterstack /skel/var/log/connector /skel/tls /skel/home/connector \
+ && chown -R 10001:10001 /skel/etc/butterstack /skel/var/log/connector /skel/tls /skel/home/connector
+
+# --- runtime ---------------------------------------------------------------
+# Default target and the one `v*` tags publish.
+#
+# glibc, not musl, and that is forced rather than preferred. The connector
+# execs the binary named by connector.yml's `perforce.binary` (default "p4",
+# internal/tools/perforce.go's exec.CommandContext), and a studio supplies
+# that binary itself -- on bsg-cp-01 by bind-mounting the p4 client out of the
+# Perforce container so the client version tracks the server automatically.
+# Perforce's packaged p4 is DYNAMICALLY linked against glibc (ldd: libc.so.6,
+# librt, libdl, libm, libpthread, /lib64/ld-linux-x86-64.so.2), so an
+# alpine/musl runtime rejects it outright:
+#
+#   alpine:3.22       -> "Dynamic loader not found: /lib64/ld-linux-x86-64.so.2"
+#   distroless/base   -> `p4 -V` runs
+#
+# Both measured against the real binary from ButterStack/perforce-debian's
+# helix-p4d package, not assumed.
+#
+# distroless/base rather than debian-slim because it supplies glibc and CA
+# certificates and nothing else: no shell, no package manager, no interpreter.
+# That matters more than usual here, since this image deliberately executes an
+# operator-supplied binary.
+#
+# Pinned by digest, which is one of the four production survival conditions
+# (#1575) the header above lists as unmet. The digest is a manifest INDEX, so
+# multi-arch still resolves; bump it deliberately, not by tag drift.
+FROM gcr.io/distroless/base-debian12:nonroot@sha256:7f0c72cd138b442ae0deeb69c08b1acf5525439ba251a49ad93c320a061567e5 AS runtime
+
+# distroless has no shell, so the account and the directories are built in a
+# stage that does, then copied in. COPY --from preserves numeric ownership.
+# uid/gid stay 10001 to match the uat stage exactly, so moving a deployment
+# between the two targets changes only the digest.
+COPY --from=skel /etc/passwd /etc/group /etc/
+COPY --from=skel /skel/ /
+
+COPY --from=build /out/butterstack-connector /usr/local/bin/butterstack-connector
+
+# No EXPOSE: the connector never listens on anything. It opens exactly one
+# outbound TLS connection and never accepts an inbound one.
+
+# Numeric on purpose: independent of /etc/passwd lookup, which a derived image
+# could overwrite.
+USER 10001:10001
+WORKDIR /home/connector
+
+ENTRYPOINT ["/usr/local/bin/butterstack-connector", "-config", "/etc/butterstack/connector.yml"]