Add dc-pull make target

I had run into strange errors with uploading files, where workhorse errored with an MD5 checksum failure when communicating with minio.

Turns out that this was because it requires minio to run in compat mode, which was fixed almost 2 years ago: https://github.com/minio/minio/pull/9326

However, I was running a 2 year old version of minio, because GCK never fetches new images, and there was no make target for it either. I think we should provide a simple solution to staying on top of container image releases.

This MR adds a make target make dc-pull; however, I wonder if we should automatically pull new images regularly somehow to discover these issues earlier?