cannot use `basic_auth`

[casperdcl]

basic_auth hashed passwords aren't properly handled:

# docker-compose.yml
  virtual.host.directives: |
    basic_auth {
      foo $2a$14$0Sp4ChWux41qQtLacUlUuOcMO7B2z1u7V2XDaKfJy0X5dgcqHglDu
    }

docker compose up
docker compose exec caddy cat /etc/caddy/Caddyfile | grep foo

WARN[0000] The "rpRTWRijdH41OrZeZvbvBunD4mkXapj3tN1qDc1sqIlTYjK1jOWaC" variable is not set. Defaulting to a blank string. 
      foo $2a$14

[casperdcl]

looks like the fix is to use double $$:

foo $2a$14$$0Sp4ChWux41qQtLacUlUuOcMO7B2z1u7V2XDaKfJy0X5dgcqHglDu

perhaps https://docs.docker.com/reference/compose-file/interpolation/ is worth documenting?

[sobolevn]

@casperdcl please, send a PR :)

[casperdcl]

Done in #95 :)