fix(decoder): ensure tag in harbor decoder

[bluebrown]

resolve: #44

[bluebrown]

Well, I added test and fixed the decoder as good as possible. If this is even required. Because opon even closer inspection, I see that the event shown here https://goharbor.io/docs/main/working-with-projects/project-configuration/configure-webhooks/, contains invalid data. They have a digest in the tag part, which is invalid according to RFC. It cant be parsed like that.

 "resources": [
      {
        "digest": "sha256:954b378c375d852eb3c63ab88978f640b4348b01c1b3456a024a81536dafbbf4",
        "tag": "sha256:954b378c375d852eb3c63ab88978f640b4348b01c1b3456a024a81536dafbbf4",
        "resource_url": "localhost/harbor/alpine@sha256:954b378c375d852eb3c63ab88978f640b4348b01c1b3456a024a81536dafbbf4"
      }
    ],

So either they have a typo on their docs, or this is broken.

decoder_test.go:78: failed to parse image ref: invalid tag: 
repository can only contain the characters `abcdefghijklmnopqrstuvwxyz0123456789_-./`:
localhost/harbor/alpine:sha256

Or they do this when no tag was pushed aka latest. This would require special logic in the decoder.