tests: make it possible to specify an alternative base domain for tests

When developing Dokku, it is useful to use a real domain to test actual functionality. This is impossible if the domain is hardcoded to dokku.me in testing.

tests/unit/docker-options.bats

@@ -333,7 +333,7 @@ teardown() {
 }
 
 @test "(docker-options) docker-options:add (all phases over SSH)" {
-  run ssh dokku@dokku.me docker-options:add $TEST_APP build,deploy,run "-v /tmp"
+  run ssh "dokku@$DOKKU_DOMAIN" docker-options:add $TEST_APP build,deploy,run "-v /tmp"
   echo "output: $output"
   echo "status: $status"
   assert_success