Jenkins: Don't install dependencies inside jobs

For now, all dependencies should be installed in the seed image in
which the job is executed, since there's no networking available
inside the job container.
Jenkins: Disable documentation publishing

Due to the lack of networking inside the job execution containers,
documentation publishing should be disabled.

See: ddb73f2c66d5c3b413363a0a23ade747f2cc7ccf
Jenkins: Specify job container seed image
Jenkins: Use pre-installed opam