Drop docker-compose

[etpinard]

We don't do anything too fancy with docker and thus docker-compose isn't the most useful thing for us.

By dropping docker-compose,

• I hope to make our image tests a little more accessible: that's one less dependency for a lot folks to install.
• In addition, our local and CI testing commands now (almost) match one-to-one - which hopefully will speed up debugging for some.
• And for bonus points, this PR removes one hard-coded file (the docker-compose.yml) lying outside our tasks/ directory.

[etpinard]

cc @alexcjohnson @mdtusz @monfera @rreusser

[mdtusz]

Seems reasonable to me.

💃