The `oci-build-task` seems to be doing more requests to the registry
than local `docker build`, which frequently results in Docker Hub's
rate limits kicking in.
By proxying `docker.io`, this should hopefully be avoided.
A somewhat elaborate setup and a custom build of `oci-build-task` are
needed: https://github.com/concourse/oci-build-task/pull/121.
Fixes: #39.
Since they I/O bound off of the same hardware, and sometimes also CPU
bound when compiling things, running them in parallel only slows
things down.
Closes: #28
As long as the image isn't used later on, there is no point in the
automatic `get` after `put`.
Also limit the inputs that each `put` step uses.
Closes: #29
