docker: add build.sh to set GIT_HASH for local builds

Simple wrapper as .git is excluded from build context.

docker/build.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+# Build the chatmail Docker image with the current git hash baked in.
+# Usage: ./docker/build.sh [extra docker-compose build args...]
+#
+# .git/ is excluded from the build context (.dockerignore) so the hash
+# must be passed as a build arg from the host.
+
+export GIT_HASH=$(git rev-parse --short HEAD)
+exec docker compose build "$@"