Create basic Dockerfile

[bbimber]

We should make a basic Dockerfile for nimble, which installs nimble and downloads the latest nimble-align. It should probably use ENTRY_POINT to point to nimble.

This workflow should virtually work as-is to make and publish a nightly docker image.

https://github.com/BimberLab/DISCVRSeq/blob/master/.github/workflows/docker.yml

Having both of these will be useful for running on exacloud.

One thing to consider: since nimble can make large temp files, the location used for those temp files should be configurable, perhaps as an argument or maybe environment variable. For docker usage, we probably do not want those written inside the container.

[hextraza]

Closed #21