Relates to https://github.com/shaarli/Shaarli/issues/843
Added:
- Makefile target to run commands in a Docker test context
- Docker images to run Shaarli test suites:
- Alpine 3.6
- Debian 8
- Debian 9
- Ubuntu 16.04
- Documentation
Signed-off-by: VirtualTam <virtualtam@flibidi.net>