The latest Debian stable images don't include the `setcap` binary by
default like they used to.
Our Docker build depends on it, so this patch makes the Dockerfile
install the libcap2-bin package (which contains the `setcap` binary).
This patch adds a new docker directory, which contains a Dockerfile plus
some additional configuration for creating a container that runs
chasquid+dovecot+letsencrypt.
It also updates the gitlab CI pipeline to automatically build and
publish an image on each commit.
This is experimental and likely to break.