Pushing back a little on silence-as-default: are we concerned about the UX of rkt "hanging" when it's not outputting anything while fetching images etc?

@jonboulle in general yes, this concern exists, which is why we'd better have some verbosity switch at some point. Another concern is that we vendor libraries out of our control, which is way rkt prepare --quiet internally masks stdout fd.

For the specific network fetching case, some bits of image&networking are left untouched here, so for example:

$ sudo run --no-store  quay.io/coreos/alpine-sh --exec echo -- "hello world!"
image: keys already exist for prefix "quay.io/coreos/alpine-sh", not fetching again
Downloading signature: [=======================================] 473 B/473 B
Downloading ACI: [=============================================] 2.65 MB/2.65 MB
image: signature verified:
  Quay.io ACI Converter (ACI conversion signing key) <[email protected]>
[19627.613248] alpine-sh[5]: hello world!

@s-urbaniak @jonboulle ping for a review. I think Jon was mostly concerned about fetching progress, but that shouldn't have changed.

As discussed oob ideally we would like to inject the lighter, but that is our of scope for this pr.

haha, autocorrection hit me ;)

s/lighter/logger

lgtm