Remove redundant dependencies from Dockerfile

A Docker build step for apt-get mono often fails on CI due to unknown reasons.
In an tentative attempt to alleviate this, we can remove unneeded apt packages and reduce download payload.

For mono:

• mono-complete was not necessary, mono-devel suffices
• we do not use xbuild and gmcs

Others:

• rake was already available without gem install rake. Also this somehow caused problem on Ubuntu Xenial.
• skip ruby doc for bundler as is often seen
• update-alternative java was for previous versions of Ubuntu and resulted in error message because no alternative is there