add argument HUGO_APPEND_PORT so user can disable it

[pdelaby]

In order to use this image with the lets-encrypt-proxy-companion, you need to disable the 'appendPort', otherwise somes features don't work (the search results are linked to my.domain.name:1313/something).

I set the default value set as true because it's the default value for it according to the documentation. So if you don't add the environment variable, nothing is changed.

[asmaloney]

Any chance we can get this merged? I'm running into the same problem.