extract first port from Dockerfile and set config variable for use in deploy phase. closes #993

tests/unit/ports.bats

@@ -108,3 +108,11 @@ teardown() {
   echo "status: "$status
   assert_success
 }
+
+@test "dockerfile port exposure" {
+  deploy_app dockerfile
+  run bash -c "grep upstream $DOKKU_ROOT/$TEST_APP/nginx.conf | grep 3000"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+}