Use docker supplied tini

[daniel-noland]

Noticed this while I was tidying up the code (I'm not trying to be nitpicky, just trying to get familiar with this code base because I plan to use it as a dependency for another project soon and I want to make sure I understand how it works).

rudolfs/Dockerfile

Lines 13 to 19 in 9f2eaad

	# Use Tini as our PID 1. This will enable signals to be handled more correctly.
	#
	# Note that this can't be downloaded inside the scratch container as we have no
	# chmod command.
	#
	# TODO: Use `--init` instead when it is more well-supported (this should be the
	# case by Jan 1, 2020).

I was going to take care of this if it is still desirable in your view @jasonwhite

I have a PR ready but it is based on top of #46 so I'll just post the PR as draft till that is merged. No need for more merge resolution noise 😄

[jasonwhite]

Yes, this change would be very welcome! I expect most distros will have a relatively recent version of Docker with the --init option now.