Add a Dockerfile so the sync server is one command to deploy

Self-hosting meant building the binary by hand and wiring Postgres up
yourself. The image is a static binary on distroless with a built-in
healthcheck flag, and docker-compose brings up Postgres alongside it.

The image itself is unbuilt here: no container runtime is available in
this environment, so only the static Linux build and the healthcheck
flag have been exercised.

💘 Generated with Crush

Assisted-by: Crush:kimi-k3

README.md

@@ -58,6 +58,9 @@ PROMPT_SYNC_DATABASE_URL="postgres://localhost:5432/prompt_sync?sslmode=disable"
   go run .          # listens on :8080
 ```
 
+Or run it with Docker (`POSTGRES_PASSWORD=… docker compose up -d`) — see
+[Server/README.md](Server/README.md) for that and for deploying to a VPS.
+
 Then in the panel: click the sync icon, set the server URL (e.g.
 `http://192.168.1.10:8080` or an HTTPS host), and either **Generate Code** on
 the first Mac or paste the code into **Join** on the other.