deploy: Dockerfile + docker-compose.yml for mDock, manual first roll

Pulls the deploy infra forward from §10 so m can see slice 1 on his LAN.

- Dockerfile: multi-stage golang:1.25-alpine → distroless/static-debian12.
  CGO_ENABLED=0 (modernc.org/sqlite is pure Go). USER 1000:1000 so the
  bind-mount on mDock (owned by m:m) is writable without chowning the
  host dir. -trimpath + -s -w; 12.2MB final image.
- docker-compose.yml: matches the mDock convention surveyed earlier
  (container_name explicit, restart: unless-stopped, env_file in
  /home/m/secrets/mcables/.env, bind-mount /home/m/stacks/mcables/data,
  port 7777 exposed on LAN). Image temporarily under the mai/ namespace
  on mgit.msbls.de because mAi doesn't have write access to m/* today —
  documented in a comment so retagging is one line when permissions land.
- .dockerignore: keeps .git, .worktrees, .m, data/, docs/, *.md,
  editor cruft out of the build context.

Manual deploy verified end-to-end:
- docker build → image sha256:76624f17 (12.2MB)
- mAi-authenticated push to mgit.msbls.de/mai/mcables:latest
- ssh mdock anonymous pull works (registry allows public reads on this
  namespace)
- POST /api/projects {"name":"LOFT"} returns the row, GET /api/projects
  shows it; docker compose restart preserves it on disk; second GET
  still shows LOFT.

Gitea Actions auto-deploy left for a follow-up task per the head's
instruction — gets us the moving parts right first.

Dockerfile

@@ -0,0 +1,36 @@
+# syntax=docker/dockerfile:1.7
+#
+# mCables — single-stage build → distroless runtime image.
+# go.mod requires go 1.25; modernc.org/sqlite is pure Go so CGO_ENABLED=0
+# and a distroless/static runtime is all we need.
+
+FROM golang:1.25-alpine AS build
+WORKDIR /src
+
+# Cache deps before copying the rest of the source.
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+# -trimpath strips local paths from the binary; -s -w drops debug info.
+RUN CGO_ENABLED=0 GOOS=linux go build \
+        -trimpath \
+        -ldflags="-s -w" \
+        -o /out/mcables \
+        ./cmd/mcables
+
+FROM gcr.io/distroless/static-debian12:nonroot
+WORKDIR /app
+COPY --from=build /out/mcables /app/mcables
+
+ENV MCABLES_ADDR=0.0.0.0:7777 \
+    MCABLES_DB=/app/data/mcables.db
+
+EXPOSE 7777
+# Run as UID:GID 1000:1000 to match m on mDock — the bind-mounted
+# /home/m/stacks/mcables/data is owned by m:m, so the container can write
+# to it without chowning the host dir. distroless/static-debian12 accepts
+# arbitrary numeric UIDs; the Go binary doesn't need a /etc/passwd entry.
+USER 1000:1000
+ENTRYPOINT ["/app/mcables"]