Merge 'swh-docker-dev' into master
Most of the files from the swh-docker-dev repo has been put in the docker/ dir
in this repository; a few files have been actually:
- merged with master's ones (namely .gitignore)
- modified (Makefile and README, docker-compose.override.yml.example),
- moved to swh-environment's root (CONTRIBUTORS and CODE_OF_CONDUCT).