Incorrect documentation examples for podman-image-diff

[infiniteregrets]

Is this a BUG REPORT or FEATURE REQUEST? (leave only one on its own line)

/kind bug

Description
In the documentation at http://docs.podman.io/en/latest/markdown/podman-image-diff.1.html
this example: podman diff --format json redis:old redis:alpine is used. But this does not work with the current version of podman.

:/$ podman image diff alpine:latest alpine:3
:/$ Error: accepts 1 arg(s), received 2 

:/$ podman diff alpine:latest alpine:3
:/$ Error: `podman diff` accepts at most one argument 

I think when a single image is mentioned the diff should be done between the image and its parent layer and when two are mentioned the diff should be between them.

The JSON output when --format=json flag is used is not indented and JSON is not accepted as a valid option, which in my opinion should be. This would just be a minor fix in the regular expression here:

podman/vendor/github.com/containers/common/pkg/report/validate.go

Line 5 in 0c38ac4

var jsonRegex = regexp.MustCompile(`^\s*(json|{{\s*json\s*(\.)?\s*}})\s*$`)

Output of podman version:

podman version 3.1.2

Please provide correct documentation examples, and if there are any potential bug fixes then let me know! I would love to try working on them

[rhatdan]

Please open a PR to fix the documentation, and if you want to add the feature to compare two images, then go for it.

[infiniteregrets]

I have already opened #10536 and it's in progress. In the meantime, i'll try working on this