Add an SSH server in each docker container.

It will not be started by default, but can be enabled by setting an
environment (ENABLE_SSH) when starting a container, and is useful for
debugging.
Rename docserver to doc for consistency.
build.sh has been superseded by Makefile.
Refactored and updated run.sh:

* support ENABLE_SSH environment variable
* start 2 keep containers (backed by the same filesystem for now)