Fix the linking of the nginx configuration file (#458)
Fix the linking of the nginx configuration file (#458)

In this commit I'm adding some logic to ensure that the `entrypoint.sh`
file handles a pre-existing `mattermost.conf` file within the nginx
folder.

During deployment of some more custom nginx configuration it was
noticed that the linking process fails, because a file already exists
with the filesystem.

Signed-off-by: Akendo <82435fe1dd1958e199afaef77fb1b3626a63938f@akendo.eu>
mattermost/mattermost-docker