Update Dockerfile and images to use more recent Ubuntu release

[zcqian]

Resolves Issue #63

In addition to updated Ubuntu version, the following changes has been made:

• Reduced packages installed: no more OpenSSH, no software-properties-common (used for apt-add-repository), no VSC Code Server
• Bumped Cerebro version
• Updated MongoDB to always use latest 4.0.x version, and Elasticsearch to always use latest 6.x version.
• A workaround was implemented and then removed for installing latest MongoDB 4.0: creating a dummy systemctl link in /usr/sbin/systemctl to /usr/bin/true. This was to prevent MongoDB post-install scripts to call the systemctl command. This need has been negated because OpenSSH server and software-properties-common are removed, hence systemd is no longer pulled into the image. Usually people create dummy packages to address issues like this, but because systemd usually depends on exact versions of a package, a dummy package that "provides" systemd does not work in this case. I'm leaving the notes here because after the commits are squashed, the workaround will not be in the git commit history.
• OpenSSH server (sshd) was previously running in the foreground in docker-entrypoint.sh to prevent this PID 1 process from exiting. Now that it has been removed, sleep infinity is invoked to prevent the script from exiting. Despite the argument "infinity" it only sleeps for a finite time, but it should be long enough to run for a few days at least (depends on the maximum value supported by time_t.
• Cleanup, so things are easier to read/edit.

[zcqian]

I have tested locally that every image should build and run fine. Someone else please review and test. After that, please squash commits and merge.

[newgene]

Add a note about the use of sleep infinity:

If this becomes an issue, e.g. the process stops at some point, an alternative could be this:

python -c 'while 1: import ctypes; ctypes.CDLL(None).pause()'

Ref: https://try2explore.com/questions/10074816