Install Dockerfile to allow containerized script execution and other improvments

See merge request !1

Dexador has implicit http pipelining which makes it faster than drakma.

Co-authored with Nicolas Dandrimont <nicolas@dandrimont.eu>.

Without this, the migration spends too much time closing unneedingly file descriptors.

Related to T4064

Some fallback uses the file cli to upload files.

Related to T4064

There exists a fallback which uses `git am`. Without this configuration, that command
explodes.

Related to T4064

This:
- explicits the necessary ssh-keygen generation
- explicits the docker/ssh volume mountpoint (as a result of ^)
- explicits the changes required in the config.lisp regarding reading the public key for
the migration to take it into account
- deactivates the known host check
- deactivates the writing to the ~/.ssh/known_hosts file to avoid out of sync
issues (when the gitlab state is reset once in a while).

Related to T4064

Through the new declared variable *rails-command* (defaulting to the previous hard-coded
ssh). This also adapts docstrings and update the generated template to clarify the use.

Related to T4064

Without this, it is expected to run the forgerie code on the same machine as the
phabricator host.

Related to T4064

Without this, the `System "forgerie"` is not found by quicklisp.

Related to T4064

Related to T4064

This speeds up re-runs without sacrificing the need to re-run when you
change what repositories you're importing

When you want to have the default project (in order to get snippets, for
instance), but don't want to apply all tickets to it because you're
doing a gradual import, you the configuration needs to respect that.

Set when the space of the phabricator project is one of the confidential
spaces.

Included is the ability to limit users added to just those that took
some action in the included repositories.

Includes:

* Add users to projects in gitlab (needed by new gitlab update)
* Fix ticket links
* Fix snippets that have attachments

This also required getting the mimetype of files and setting them
correctly.