Some Dockerfile improvements (#3182)

- improve docker_entrypoint.sh
- serve static files with puma by default
- sort packages list
- use virtual package for build deps
- show how to assign UID/GID

.env.production.sample

@@ -95,3 +95,8 @@ SMTP_FROM_ADDRESS=notifications@example.com
 # Cluster number setting for streaming API server.
 # If you comment out following line, cluster number will be `numOfCpuCores - 1`.
 STREAMING_CLUSTER_NUM=1
+
+# Docker mastodon user
+# If you use Docker, you may want to assign UID/GID manually.
+# UID=1000
+# GID=1000