fix: Correctly set up dockerignore file

* Rename file to `Dockerfile.dockerignore`. As the `Dockerfile` is not
  in the root directory, the Docker spec requires the file to be
  named `<dockerfile_name>.dockerignore`.
* Update ignore rules, and include `.mypy_cache/` directory.

docker/.dockerignore

@@ -1,23 +0,0 @@
-**/node_modules
-node_modules/
-
-**/venv
-venv/
-
-**/__pycache__
-__pycache__/
-
-**/assets/library
-../frontend/assets/library/
-
-**/.env
-.env
-
-**/romm_test
-romm_test/
-
-**/romm_mock
-romm_mock/
-
-**/.pytest_cache
-.pytest_cache/

docker/Dockerfile.dockerignore

@@ -0,0 +1,18 @@
+# Configuration
+**/.env
+
+# Frontend
+**/assets/library/
+**/node_modules/
+
+# Backend
+backend/tests/
+**/venv/
+**/.venv/
+**/.mypy_cache/
+**/.pytest_cache/
+**/__pycache__/
+
+# Testing
+**/romm_test/
+**/romm_mock/