Add nil check for current docker context

[eharris128]

What

• Remove assumption that user system has a docker config file

Why

• Prevent panic on invocation of mint images

Initial Bug Reproduction Steps

• On latest master, invoke make build
• Have a fresh install of docker on LTS Ubuntu 22.04 - resulting on no user configured options for docker.
  

The panic:

How Tested

• make build
• mint images --runtime=docker
• Confirm you get no panic

[eharris128]

Consequence of not performing the other work this PR leaves out:

Maybe the onus is on the user?

[kcq]

Consequence of not performing the other work this PR leaves out:

Maybe the onus is on the user?

Need to fail more gracefully either way... Don't need any credentials to pull the "busybox@sha256:05..." image. Also not quite sure where the "invalid reference format" API error is coming from.

[kcq]

Consequence of not performing the other work this PR leaves out:

Maybe the onus is on the user?

@eharris128 looks like we need a couple of enhancements here... First, the getDockerCredential - failed to acquire local docker config path warning login needs an extra check to see if the error is no docker credentials provider found (there's no error var defined for it in go-dockerclient unfortunately). The no docker credentials provider found is ok because it just means there's no credentials provider for the target registry, which happens all the time. If it's that error than we don't need to log the warning (pkg/crt/docker/dockercrtclient/dockercrtclient.go#L294) and we can do the same thing as if we don't find the auth config.

The image.inspector.Pull warning shows that repo is busybox@sha256 and tag is 05a79..., which explains that invalid reference format API error. The pull logic in the image inspector needs to be enhanced to parse image paths with digests properly. Right now it simply splits on : expecting name:tag, but it needs to check if we have a digest by checking for @ and then split on @ when we have name@algorithm:hashvalue.

[eharris128]

For completeness - ran into the similar config enhancement opportunity with k8s in my testing last night.

Podman / containerd I did not test to see their behavior.