Allow host+subpath as the source registry for registry-override

[lsoica]

Currently, the --registry-override will respect the subpath when going from a host to a host+subpath but not the other way around.
(so --registry-override docker.io=mydomain.com/subpath works but --registry-override docker.io/subpath=mydomain.com doesn't)

The ask here is to extend the functionality with support for:

• --registry-override docker.io/subpath=mydomain.com
• --registry-override docker.io/subpath=mydomain.com/anothersubpat

Additional context

Slack thread https://kubernetes.slack.com/archives/C03B6BJAUJ3/p1699559557724489?thread_ts=1687158762.361229&cid=C03B6BJAUJ3

[waveywaves]

Hi team, I tried reproducing this issue. Was unsuccessful in doing so.
I took the following steps to reproduce this issue.

change pwd to one of the examples

cd examples/docs-games

update the zarf.yaml images spec image to contain docker.io
so that the configuration looks like the following

images:
- docker.io/defenseunicorns/zarf-game:multi-tile-dark

pull, re-tag and push this image to personal docker registry at docker.io/waveywaves/zarf-game:multi-tile-dark

override registry

zarf create package --registry-override docker.io/defenseunicorns=docker.io/waveywaves

the above command runs successfully and so do the ones below

zarf create package --registry-override whatever.io=docker.io/defenseunicorns
zarf package create --registry-override whatever.io/whatevs=docker.io

Would appreciate any direction understanding how to reproduce this issue
As per the slack thread I am expecting to see the following message in the CLI

Falling back to docker for whatever.io/whatevs/repository/image:tag

[Racer159]

@waveywaves the way @lsoica ran it was a bit different, mostly showing how a replacement happened or didn't happen; with just a host the replacement will happen with whatever you put in the = so:

zarf package create examples/dos-games/ --registry-override docker.io=localhost:555/hello

Works as expected, but:

zarf package create examples/dos-games/ --registry-override docker.io/defenseunicorns=localhost:555

Does not work and will proceed as normal (in @lsoica 's case keeping dummy.remap and not using fqdn.ext at all, and in yours ignoring the replacement and continuing to success).

The problem comes down to this part in the code:

zarf/src/internal/packager/images/pull.go

Line 87 in 7e91d3b

if overrideHost, present := i.RegistryOverrides[refInfo.Host]; present {

We only look for refInfo.Host in the override map when we should instead be looking for a string prefix to the entire refInfo.Reference. That function should also not use ImageTransformHostWithoutChecksum since we already have the parsed ref and all the info we need to do the replacement (and need to perform the replacement differently than that function expects.

Instead of doing that if statement it should loop through the keys and values in i.RegistryOverrides, check if the refInfo.Reference begins with an override key and, if it does, replace that override text with the override value and set it back to actualSrc.