Improve Dockerfile generation

[smondet]

Still to do:

• [x] need to wait for the hammerlab/keredofi:ubuntu-docker tag to be built; and use it for the tests
• [ ] use (ppx_deriving_)cmdliner (?) (will do later)
• [x] try hammerlab/keredofi:secotrec-default; use it in README.md; remove Dockerfile and Hub build.

[smondet]

[smondet]