build: add a compose file that builds the image locally

docker-compose.yml pulls registry.reversed.dev. This variant builds from the
checkout instead and needs no .env — every variable falls back to a working
localhost default, so the stack comes up ready to register the first admin.

Also exposes Postgres on 5434 (clear of the dev database on 5433) and adds a
backend healthcheck so `depends_on` ordering is meaningful.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -117,7 +117,8 @@ Backend/
     migrations/
 UI/               React + Tailwind frontend (Vite), builds to UI/build/
 Dockerfile        multi-stage build (UI then backend)
-docker-compose.yml  backend + Postgres
+docker-compose.yml        backend (pre-built image) + Postgres
+docker-compose.local.yml  same stack, built from this checkout
 example.env
 .gitea/workflows/deploy.yml
 ```
@@ -155,6 +156,26 @@ docker compose up -d
 # Open http://localhost:5000 — the first account to register becomes admin.
 ```
 
+### Building the image locally
+
+To run from this checkout instead of the pre-built image — no `.env` needed, every
+variable falls back to a working localhost default:
+
+```bash
+docker compose -f docker-compose.local.yml up -d --build
+```
+
+BuildKit builds both Dockerfile stages in parallel, which can exceed Docker
+Desktop's memory limit during `pnpm install`. If the build fails with "cannot
+allocate memory", warm the first stage on its own and retry:
+
+```bash
+docker build --target build -t patchpass-build .
+```
+
+> The local compose file ships a placeholder `INSTANCE_SECRET`. Override it before
+> using the instance for anything real.
+
 ---
 
 ## Development setup