Support images with more than one forward slash

[rchui]

This PR address #348.

Explanation (https://regex101.com/r/aPUxUr/1):

^(.+?(?:[:.].+?)\/)?(.*?)(?:[@:])?((?<=[@:]).*)?$

        Explanation:

            (.+?(?:[:.].+?)\/)? - [GROUP 0] REGISTRY
                .+?                 - A registry must start with at least one character
                (?:[:.].+?)\/       - A registry must have ":" or "." and end with "/"
                ?                   - Make a registry optional
            (.*?)               - [GROUP 1] REPOSITORY
                .*?                 - Get repository name until separator
            (?:[@:])?           - SEPARATOR
                ?:                  - Don't capture separator
                [@:]                - The separator must be either "@" or ":"
                ?                   - The separator is optional
            ((?<=[@:]).*)?      - [GROUP 2] TAG / DIGEST
                (?<=[@:])           - A tag / digest must be preceeded by "@" or ":"
                .*                  - Capture rest of tag / digest
                ?                   - A tag / digest is optional

        Examples:

            image
                - None
                - image
                - None
            example/image
                - None
                - example/image
                - None
            example/image:tag
                - None
                - example/image
                - tag
            example.domain.com/example/image:tag
                - example.domain.com/
                - example/image
                - tag
            123.123.123.123:123/example/image:tag
                - 123.123.123.123:123/
                - example/image
                - tag
            example.domain.com/example/image@sha256:45b23dee0
                - example.domain.com/
                - example/image
                - sha256:45b23dee0

[corleyma]

I think this looks good and should match up with how the open source docker registry treats this (using this code as my comparison for source of truth).