It can be convenient to upload images to dockerhub for redundancy and
visibility, so this patch updates the gitlab CI configuration to do
that.
While at it, rename the stages for readability.
It is useful to run the integration tests both against the submitted
dependencies, based on Go module versions from the repository, as well
as the latest ones, to catch integration problems early.
This patch extends the .gitlab-ci.yml configuration to do that, by
optionally passing a flag to "go get" to update the modules.
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.
