Fix broken block parsing when label prefix contains dots

[Vigilans]

When CADDY_DOCKER_LABEL_PREFIX contains dots, parsing of blocks will break.

e.g. With CADDY_DOCKER_LABEL_PREFIX=mydomain.gitlab.caddy, cdp will yield error:

[ERROR]  Removing invalid block: Caddyfile:2: unrecognized global option: gitlab\n{\n\tgitlab {\n\t\tcaddy http://redis.gitlab.mydomain {\n\t\t\treverse_proxy 127.0.0.1:5540\n\t\t}\n\t}\n}\n\n

Then wrongly parsed block is:

{
        gitlab {
                caddy http://redis.gitlab.mydomain {
                        reverse_proxy 127.0.0.1:5540
                }
        }
}

It is because gitlab.caddy in mydomain.gitlab.caddy also gets parsed as path.

Using dots in prefix conforms to what docker has adopted in their labels, e.g.:

            "com.docker.compose.project": "...",
            "com.docker.compose.project.config_files": "...",
            "com.docker.compose.project.working_dir": "...",

So using dot-separated string in CADDY_DOCKER_LABEL_PREFIX should be a good way to keep naming style consistent with other labels and should be supported.

This PR fixes it by canonicalizing the label prefix to "caddy", so meta characters in user provided label prefix will not affect cdp's block parsing.

[Vigilans]

Here uses regexp.QuoteMeta to escapes all regular expression metacharacters in the user-provided label prefix, to make it a literal text in the labelRegex.

[Vigilans]

@lucaslorentz @francislavoie Ping for a review, I've used it for months and it proves to work usefully.

Use cases:

services:
  cs2-ingress:
    image: .../caddy:latest # with caddy-docker-proxy built
    environment:
      CADDY_DOCKER_LABEL_PREFIX: game.cs2.caddy
    labels:
      game.cs2.caddy: http://
      game.cs2.caddy.handle_path: /fastdl/*
      game.cs2.caddy.handle_path.file_server: browse
      game.cs2.caddy.handle_path.file_server.root: /srv/www


  ds3:
    image: timleonarduk/ds3os:latest
    labels:
      game.ds3.caddy: http://
      game.ds3.caddy.reverse_proxy: 127.0.0.1:50005

services:
  gitlab-postgres-webui:
    image: sosedoff/pgweb:latest
    labels:
      nas.gitlab.caddy: http://postgres.gitlab.mydomain
      nas.gitlab.caddy.reverse_proxy: 127.0.0.1:5433

  harbor-redis-webui:
    image: ghcr.io/joeferner/redis-commander:latest
    labels:
      nas.harbor.caddy: https://redis.harbor.mydomain
      nas.harbor.caddy.import: tls
      nas.harbor.caddy.reverse_proxy: 127.0.0.1:6380

nas.gitlab.caddy prefix is better than nas-gitlab-caddy because it is in consistence with labels like com.docker.compose.