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

Server/.dockerignore

@@ -0,0 +1,4 @@
+.git
+prompt-sync
+prompt-sync-*
+*.md