fix(webhook) Fix regexp

[coffeecupjapan]

Thank you for reading my PR.

What?

According to kubernete's DNS validation code here, valid way to validate dns name should be

	dnsLabelFormat                 = `[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?`
	dnsSubdomainFormat             = fmt.Sprintf(`^%s(?:\.%s)*$`, dnsLabelFormat, dnsLabelFormat)
	validWindowsUserDomainDNSRegex = regexp.MustCompile(dnsSubdomainFormat)

I wonder we should remove _ / should add ending [a-zA-Z0-9] and more small fixes or not, but in this case I just add heading [a-zA-Z0-9] to prevent

`.somedns.com`
`-somedns.com`
`_somedns.com`

to be matched.

Or we can just imitate original kubernete code may be.

Any suggestions are welcomed. Thanks.

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[prometherion]

Hey @coffeecupjapan, thanks for your contribution, I really appreciate it!

In regard to regex, we don't have proper unit tests and your suggested regex could be potentially broken, as shown here for .somedns.com/clastix/capsule:v1.0.0.

This is definitely an edge case since it's broken: it would be great if we could have a regex that compiles only with a valid fully qualified container reference.

[coffeecupjapan]

@prometherion
Thank you for your reply.

I want to confirm where this regex is broken.
For .somedns.com/clastix/capsule:v1.0.0 case registry group is not matched and code here just throws error, and I think this behavior is something expected.

Should registry for .somedns.com/clastix/capsule:v1.0.0 be valid?

Maybe I am wrong somehow, so I want to hear what I am wrong for this regexp.
Sorry for my lack of consideration.

Thank you.

[prometherion]

No need to be sorry, @coffeecupjapan :)

Any enhancement or improvement here is highly welcome! As I said, I'm not the regex expert, if you think we could enhance it I'll be happy to test it and get it merged, otherwise, let's close this one to avoid having stale PR.

Just a friendly reminder, we also have a #capsule channel for direct interactions, happy to see you there!

[coffeecupjapan]

@prometherion
Thank you!
Let us discuss this topic at Slack!