docker: Add Dockerfile for running chasquid+dovecot+letsencrypt

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.

.mkdocs.yml

@@ -25,6 +25,7 @@ nav:
     - hooks.md
     - dovecot.md
     - dkim.md
+    - docker.md
     - flow.md
     - monitoring.md
     - sec-levels.md