Fix docker-entrypoint when called with atlantis (runatlantis#3060)

Prior to v0.22.0 the container could be run with the command `atlantis
server`. However this was broken by runatlantis#2770:
```
Error: unknown command "atlantis" for "atlantis"
```

This PR reverts to previous functionality and matches the comment.
Following calls all work again:
```sh
docker run atlantis server
docker run atlantis --help
docker run atlantis atlantis server
docker run atlantis atlantis --help
```

docker-entrypoint.sh

@@ -5,7 +5,7 @@ set -e
 
 # If the user is trying to run atlantis directly with some arguments, then
 # pass them to atlantis.
-if [ "$(echo "${1}" | cut -c1)" ]; then
+if [ "$(echo "${1}" | cut -c1)" = "-" ]; then
     set -- atlantis "$@"
 fi