Enforce environment variable configuration for Docker entrypoint

- Updated `docker-entrypoint.sh` to disallow custom command-line arguments, requiring configuration through `SCRATCHBOX_*` environment variables.
- Modified `README.md` to clarify that custom container commands are not supported and to reflect the new entrypoint behavior.
- Enhanced `main_test.go` to validate failure when required environment variables are not set.
- Added a function in `server.go` to print startup environment variables, ensuring visibility of configuration during server initialization.

docker-entrypoint.sh

@@ -13,12 +13,10 @@ export SCRATCHBOX_SERVER_LISTEN_ADDR
 SCRATCHBOX_SERVER_ACCESS_LOG_FILE_PATH=
 export SCRATCHBOX_SERVER_ACCESS_LOG_FILE_PATH
 
-# Default to running the server from SCRATCHBOX_* env vars.
-if [ "$#" -eq 0 ]; then
-  set -- server --env
-elif [ "${1#-}" != "$1" ]; then
-  # Allow passing server flags directly, e.g. `docker run ... --env`.
-  set -- server --env "$@"
+# Entrypoint is fixed for Docker deployments; custom args are not supported.
+if [ "$#" -ne 0 ]; then
+  echo "custom entrypoint arguments are not supported; configure via SCRATCHBOX_* env vars" >&2
+  exit 2
 fi
 
-exec scratchbox "$@"
+exec scratchbox server --env