Support escaping dots in label keys with \.

[lucaslorentz]

Summary

• Adds support for \. as an escaped literal dot in Docker label keys
• The . character is used as a nesting separator, which prevented using literal dots in directive names (e.g. domain names like mydomain.fr or Caddy's {blocks.name} syntax)
• \. is replaced before regex parsing and restored as . in the directive name

Examples

Caddy blocks syntax (#767):

caddy.reverse_proxy.{blocks\.proxy_target} =

produces:

reverse_proxy {
    {blocks.proxy_target}
}

Domain names (#699):

caddy.dynamic_dns.domains.mydomain\.fr =

produces:

dynamic_dns {
    domains {
        mydomain.fr
    }
}

Fixes #767
Fixes #699

Test plan

• All existing tests pass
• New test case escaped_dots.txt covers both use cases

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