Improve docker setup

- add .dockerignore
- add entrypoint.sh to switch to user after initial setup
- downgrade importlib-metadata for Celery compatibility
- add TZ environment variable for consistent Celery timestamps
- fix Celery setting deprecation warnings

.dockerignore

@@ -0,0 +1,17 @@
+**/*.pyc
+**/.*.swp
+**/.coverage
+**/__pycache__
+.dockerignore
+.env
+.envrc
+.git
+.gitignore
+.idea
+.vagrant
+Dockerfile
+Vagrantfile
+change-vmdebootstrap-default-dhcp.sh
+coverage-report
+docs
+salt