implement `opam-ci docker`

[avsm]

The opam-ci logs command currently outputs a Dockerfile which has to be manually cut-and-pasted and built. Instead, this functionality should be shifted into an opam-ci docker subcommand which can:

# generate an interactive container
opam-ci docker run xen-evtchn.1.0.3 --compiler=4.06 --distro=debian-9
# build an image called `local-container`
opam-ci docker build -t local-container xen-evtchn.1.0.3 --compiler=4.06 --distro=debian-9
# just output the Dockerfile to stdout
opam-ci docker file xen-evtchn.1.0.3 --compiler=4.06 --distro=debian-9 

[avsm]

We could also do ssh agent forwarding and volume mount the local git setup. @djs55 suggests looking for ~/Library/Containers/com.docker.docker/Data to detect Docker for Mac (since Linux doesnt do the right uid translation we need)