Optimize CI and remove deployment steps, add Mattermost and update Etherpad

1. Remove intermediate image

We use a base image for all steps of the CI (containing git, wget, ...) and use the Gitlab cache system rather than environment variables in a specific image to pass needed values across jobs, such as the name of the image to build and to analyze.

2. Generalize CI

The image will be triggered for all Dockerfile and docker-compose.yml files.

3. Remove deployment

Due to security and usability reasons, deployment has been removed. As a consequence :

• Secrets are no longer managed by Gitlab / PGP
• Certificates to access the docker socket are no longer exposed on Gitlab
• Ad-hoc scripts to define which service belongs to which host are removed

So, the image is built, analyzed and pushed only.

4. Updated images

Etherpad is updated to the newest version and Mattermost is now a custom image managed by the CI.

5. Various improvements

Factorization in .gitlab-ci.yml