Drop prebuilt images; allow build without `executors/docker/bindata.go`

Using pre-built images incorporated into executors/docker/bindata.go strikes me as incredibly sloppy.
Also this approach is incompatible with Debian policy.

Downloading on build-time is prohibited in most (sane) build environments.

It should be possible to build GitLab Runner without bindata.go,
with proper dependencies on Docker (and other packages/daemons) as required.