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

docker_entrypoint.sh

@@ -1,4 +1,14 @@
 #!/bin/sh
+
+### 1. Adds local user (UID and GID are provided from environment variables).
+### 2. Updates permissions, except for ./public/system (should be chown on previous installations).
+### 3. Executes the command as that user.
+
+echo "Creating mastodon user (UID : ${UID} and GID : ${GID})..."
 addgroup -g ${GID} mastodon && adduser -h /mastodon -s /bin/sh -D -G mastodon -u ${UID} mastodon
+
+echo "Updating permissions..."
 find /mastodon -path /mastodon/public/system -prune -o -not -user mastodon -not -group mastodon -print0 | xargs -0 chown -f mastodon:mastodon
-su-exec mastodon:mastodon /sbin/tini -- "$@"
+
+echo "Executing process..."
+exec su-exec mastodon:mastodon /sbin/tini -- "$@"