Significantly improve the Dockerfile

[nhooyr]

• Adds dumb-init so closes Use an init system #407, closes docker one-liner should use the -i flag #361, closes Use a tinyinit in the dockerfile #383
• User mode docker so closes feat: user-mode docker #192, closes sudo in Dockerfile? #65
• Uses latest docker ubuntu instead of 18.10 which is the rolling tag so
  closes Switch to an LTS version of Ubuntu (18.04) for the Dockerfile  #404

Thanks to @RichardMcSorley and @sr229

Describe in detail the problem you had and how this PR fixes it

Is there an open issue you can link to?

[nhooyr]

Will need to update the README.md with a command to mount into /home/coder/project.

[nhooyr]

Unfortunately, dumb-init is not sending signals properly when using the -t option to docker so I'm not sure what's going on.

[nhooyr]

Nvm, got it working. See https://stackoverflow.com/questions/41097652/how-to-fix-ctrlc-inside-a-docker-container

[RichardMcSorley]

@nhooyr Great you got it working.

[frol]

Passwordless sudo is like an always-open door (in the real world). There is definitely a room for improvement.

[sr229]

@frol its still teaching basic practice anyways, so I don't think primarily its open door especially if its just a container (and the fact GCP practices this as well due to the fact - you know, they're SSH keys only by default).

[nhooyr]

We could prompt users to set a password on startup later @frol but for now this is a simple improvement.

[sr229]

I vote against that IMHO, I don't think we need to support that kind of use case especially in Docker. The user should do it themselves if they really want to. We just provide a sane default for them.

[nhooyr]

Asking for a password is a safe default and the common use case. What we're providing is the exception.