Add oot-dev and pybombs-dev images

[igorauad]

The idea is based on gnuradio/gnuradio#6194. I've been maintaining the igorfreire/gnuradio-oot-dev image on DockerHub and using it for various things, like testing OOTs on CI (see the gr-dvbs2rx CI). So the idea is to migrate this image into the gnuradio/ namespace on DockerHub.

Issue gnuradio/gnuradio#6194 mentions creating a dedicated repo for this, but I have the sense that a single repository handling all GR-related Docker images would work better. From a quick look, it seems this repository (gnuradio-docker) would be the appropriate place.

This PR changes the following:

• Renames and moves the Dockerfiles used on GNU Radio CI, which are now called CI builders. With this PR, the repo would contain Dockerfiles for multiple purposes, and I've organized all dockerfiles into a single docker/ directory. All files have the .dockerfile extension and the CI builders are named as ci-*.dockerfile.
• Adds the oot-dev image and extensive documentation adapted from https://github.com/igorauad/docker-gnuradio-oot-dev/blob/master/README.md.
• Adds the pybombs-dev image based on the igorfreire/pybombs-dev image proposed in Add workflow to test building changed recipes gr-etcetera#121. I recognize the pybombs-docker repo contains similar images and would be happy to adapt here based on them if you prefer.

If you agree with the changes, the roadmap for the PR would be to:

• Confirm what to do about the PyBOMBS development image.
• Make sure the upload jobs work when uploading to the gnuradio/ DockerHub account. They were tested in my account only.