conformance: skip check (by default) for zero layers in image manifest

[rchincha]

As per image-spec, since this is a SHOULD (hence recommended) ...

https://raw.githubusercontent.com/opencontainers/image-spec/main/manifest.md

===

layers array of objects

Each item in the array MUST be a descriptor. For portability, layers SHOULD have at least one entry.

===

Fixes #410

[sajayantony]

I really hope to see zero layers come back as a valid manifest. Is it possible to keep these cases to see in the future which registries support these without failing conformance? This matrix should be automatable - https://github.com/opencontainers/image-spec/issues/1025

[rchincha]

Perhaps, a way out could be to just enforce all MUSTs and warn on all SHOULDs, so we know without breaking the spirit of the conformance tests.

IINM, this particular test was enforcing that registries that don't accept zero layer manifests are not compliant, whereas the image-spec language has no such language. Also [this](https://github.com/opencontainers/image-spec/blob/main/schema/image-manifest-schema.json#LL30C1-L30C1} needs updating to 0 perhaps.

[rchincha]

A simpler variant of what we want to achieve.

[rchincha]

Updated the PR with one more commit for a different option.

[rchincha]

@sudo-bmitch et al, the two commits can be squashed if we agree with the warning approach.

[rchincha]

Borrowed the Warn() from https://github.com/opencontainers/distribution-spec/pull/375, also what appears to be a bugfix

[rchincha]

zot/main passes with these changes

••••••••••••••••HTML report was created: /local/rchincha/go/src/github.com/opencontainers/distribution-spec/conformance/report.html JUnit report was created: /local/rchincha/go/src/github.com/opencontainers/distribution-spec/conformance/junit.xml

Ran 63 of 68 Specs in 0.204 seconds SUCCESS! -- 63 Passed | 0 Failed | 0 Pending | 5 Skipped

[rchincha]

zot/main instrumented to cause warn passes but with warning

+++ b/pkg/storage/common/common.go
@@ -94,6 +94,10 @@ func ValidateManifest(imgStore storageTypes.ImageStore, repo, reference, mediaTy
                                return "", zerr.ErrBadManifest
                        }
                }
+
+               if len(manifest.Layers) == 0 {
+                       return "", zerr.ErrBadManifest
+               }

---

•• WARNING: image manifest with no layers is not supported

/local/rchincha/go/src/github.com/opencontainers/distribution-spec/conformance/02_push_test.go:366                                                                    ••••••••

---

••••••••••••••••HTML report was created: /local/rchincha/go/src/github.com/opencontainers/distribution-spec/conformance/report.html JUnit report was created: /local/rchincha/go/src/github.com/opencontainers/distribution-spec/conformance/junit.xml

Ran 63 of 68 Specs in 0.186 seconds SUCCESS! -- 63 Passed | 0 Failed | 0 Pending | 5 Skipped