docker: introduce multi-stage image build (master, latest)

docker: introduce multi-stage image build (master, latest)

Relates to https://github.com/shaarli/Shaarli/issues/755
Relates to https://github.com/shaarli/Shaarli/pull/1072

See:
- https://docs.docker.com/develop/develop-images/multistage-build/
- https://hub.docker.com/r/library/composer/
- https://github.com/composer/docker
- https://github.com/docker-library/docs/tree/master/composer

Signed-off-by: VirtualTam <virtualtam@flibidi.net>