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.

docker/README.md

@@ -0,0 +1,84 @@
+
+# Docker
+
+chasquid comes with a Dockerfile to create a container running [chasquid],
+[dovecot], and managed certificates with [Let's Encrypt].
+
+**IT IS EXPERIMENTAL AND LIKELY TO BREAK**
+
+The more traditional setup is **highly recommended**, see the
+[how-to](howto.md) documentation for more details.
+
+[chasquid]: https://blitiri.com.ar/p/chasquid
+[dovecot]: https://dovecot.org
+[Let's Encrypt]: https://letsencrypt.org
+
+
+## Images
+
+There are [pre-built images at gitlab
+registry](https://gitlab.com/albertito/chasquid/container_registry).  They are
+automatically built, and tagged with the corresponding branch name. Use the
+*master* tag for a stable version.
+
+If, instead, you want to build the image yourself, just run:
+
+```sh
+$ docker build -t chasquid -f docker/Dockerfile .
+```
+
+
+## Running
+
+First, pull the image into your target machine:
+
+```sh
+$ docker pull registry.gitlab.com/albertito/chasquid:master
+```
+
+You will need a data volume to store persistent data, outside the image. This
+will contain the mailboxes, user databases, etc.
+
+```sh
+$ docker volume create chasquid-data
+```
+
+To add your first user to the image:
+
+```
+$ docker run \
+	--mount source=chasquid-data,target=/data \
+	-it --entrypoint=/add-user.sh \
+	registry.gitlab.com/albertito/chasquid:master
+Email (full user@domain format): pepe@example.com
+Password:
+pepe@example.com added to /data/dovecot/users
+```
+
+Upon startup, the image will obtain a TLS certificate for you using [Let's
+Encrypt](https://letsencrypt.com/). You need to tell it the domain(s) to get a
+certificate from by setting the `AUTO_CERTS` variable.
+
+Because certificates expire, you should restart the container every week or
+so. Certificates will be renewed automatically upon startup if needed.
+
+In order for chasquid to get access to the source IP address, you will need to
+use host networking, or create a custom docker network that does IP forwarding
+and not proxying.
+
+Finally, start the container:
+
+```sh
+$ docker run -e AUTO_CERTS=mail.yourdomain.com \
+	--mount source=chasquid-data,target=/data \
+	--network host \
+	registry.gitlab.com/albertito/chasquid:master
+```
+
+
+## Debugging
+
+To get a shell on the running container for debugging, you can use `docker ps`
+to find the container ID, and then `docker exec -it CONTAINERID /bin/bash` to
+open a shell on the running container.
+