Add a dockerfile, end-to-end sanity test

[OJFord]

Could potentially form the basis of (or simplify) more testing involving environment variables and filesystem setup too.

Closes #241

[OJFord]

I didn't add anything around pushing the resultant image, should be pretty straightforward (especially to GitHub's registry), just needs you to add secret anyway so left it for now.

more testing involving environment variables and filesystem setup

or bash versions.