Dockerfile: run asset precompilation as a build step (#7780)

* Dockerfile: run asset precompilation as a build step

* Replace OTP_SECRET and SECRET_KEY_BASE with placeholders

Dockerfile

@@ -77,8 +77,10 @@ COPY . /mastodon
 
 RUN chown -R mastodon:mastodon /mastodon
 
-VOLUME /mastodon/public/system /mastodon/public/assets /mastodon/public/packs
+VOLUME /mastodon/public/system
 
 USER mastodon
 
+RUN OTP_SECRET=precompile_placeholder SECRET_KEY_BASE=precompile_placeholder bundle exec rails assets:precompile
+
 ENTRYPOINT ["/sbin/tini", "--"]