Fix docker build & run

[exhuma]

This fixes #10

To make everything work I needed to make three changes (explained below). I would say that all three are beneficial for the project itself:

1. Add the name option to the Pipfile sources.
   This was required to make pipfile lock run through without error.
2. Install an officially packaged version of pyyaml instead of relying on the latest git branch
   I assume that installing from git was done to get around the recent issue of installing pyyaml. With an updated Pipfile.lock this workaround is no longer necessary.
3. Update the lock file
   With an updated Python version, and the fixed pyyaml version we can use updated versions of everything.

[exhuma]

I'm not quite sure what happened to the checks. Seems like the macOS build has timed out? It simply says Error: The operation was canceled.

Any idea what I could do to fix that?

[kevin1024]

Some of the test runs have been a bit flaky. Not sure what the deal is. Thanks for your PRs though!