Docker-sonarqube: Provide a docker image for SonarQube community edition

[janos-ss]

See also https://jira.sonarsource.com/browse/MMF-1449

Sub-tasks

Required to create public PRs:

• [x] Define images with appropriate aliases (6.7.6-community, 6.7-community, lts, 7.4-community, latest)
  
  
  
  • https://github.com/docker-library/docs/pull/1367
  
  
  • https://github.com/docker-library/official-images/pull/5098
  
  
• [x] Create simple automated tests for the images (get inspired by Elastic)
• [ ] (optional) Create automated tests as recommended in official images repo
• [x] Make it possible to pass all sonar. parameters using Docker env variables
• [x] Document how to use parameters via Docker environment variables (also, adopting Elastic's approach to documentation)
• [x] Document how to use bind-mounted persistent volumes
• [x] Document how to create custom images containing your configuration and plugins

SonarQube public documentation:

• [x] Document how to use Docker in Get started in 2 minutes
• [x] Document how to use Docker in Installing the server

SonarSource internal documentation, etc:

• [x] Document how to release a new version
• [x] Add to SonarQube release process a step to update Docker images
• [x] Add open PRs to Platform Team's wallboard widget
• [x] (optional) Do a triage of existing GitHub PRs
• [x] (optional) Do a triage of existing GitHub Issues
• [x] (optional) Notify current threads of the availability of the new images and docs

[janos-ss]

The new images were released. You can give them a spin with, for example:

docker pull sonarqube
docker run -p 9000:9000 sonarqube  # runs latest: 7.4
docker run -p 9000:9000 sonarqube:6.7.6-community