CI support for Domainr

At the moment, this is just a Dockerfile, used to create domainr/ci on Docker Hub as a public image.

Nothing proprietary or secret goes in this image.

We want to get new stable releases of Go quickly, so use the Golang upstreams which are fast enough, then add in whatever other packages and tools we expect.

We include the docker client, to work with Circle CI’s setup_remote_docker (where a container talks to docker to create images from inside docker).

We also include heroku, dep and various other things.

We build latest from master, but we also include a generational pseudo-latest tag, named for a lake in Montana. If experiments with a branch build reveal something which would break existing builds if merged to master, and we're lucky enough to spot those problems, then we can update the lake name in Docker Hub before the merge, so that future builds get a new lake name and dependencies can update independently.

The first is swan, in case this experiment takes a swan dive. Next: Bowman Lake in Glacier National Park.

We also git-tag some releases with Go version numbers, so if latest or $lake is broken by a bad merge, we can at least re-docker-tag that back onto the docker-tag from that git-tag and get something working, even if not the most recent.

We split the Dockerfile into two stages, so that it's easy to create the "still running as root" image.

You can also use a local HTTP caching proxy, if you want to reduce network usage and fetch apt packages via that. This is a Predefined ARG of Docker.

Thus:

% docker build --build-arg http_proxy=http://192.0.2.1:3128/ \
    -t foo-root --target rootstage .

% docker build --build-arg http_proxy=http://192.0.2.1:3128/ \
    -t foo .

% docker run -it --rm foo       # do things without root
% docker run -it --rm foo-root  # do things with root