docker: slim build by excluding .git and non-essential files

Replace the in-Dockerfile `git rev-parse HEAD` with a GIT_HASH build arg
passed from docker-compose (local) or github.sha (CI), defaulting to
"unknown" when unset.

Also exclude .github/, docs/, tests/, and *.md (except www/**/*.md).

docker-compose.yaml

@@ -12,6 +12,8 @@ services:
     build:
       context: ./
       dockerfile: docker/chatmail_relay.dockerfile
+      args:
+        GIT_HASH: ${GIT_HASH:-unknown}
     image: chatmail-relay:latest
     restart: unless-stopped
     container_name: chatmail