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.
