fix: detect correct python binary to use

Ideally python usage goes away - the dokku-installer should become a mandatory cli command, and the plugin is rewritten in golang - but for now, we need to use the correct binary when running dokku.

Closes #3714

plugins/scheduler-docker-local/scheduler-inspect

@@ -48,7 +48,15 @@ EOF
   for CONTAINER_FILE in $CONTAINER_FILES; do
     CIDS+="$(<"$DOKKU_ROOT/$APP/$CONTAINER_FILE")"
   done
-  "$DOCKER_BIN" container inspect "${CIDS[@]}" | python2.7 "$TMP_INSPECT_CMD"
+
+  local PYTHON_BIN=python
+  if command -v "python3" &>/dev/null; then
+    PYTHON_BIN=python3
+  elif command -v "python2.7" &>/dev/null; then
+    PYTHON_BIN=python2.7
+  fi
+
+  "$DOCKER_BIN" container inspect "${CIDS[@]}" | "$PYTHON_BIN" "$TMP_INSPECT_CMD"
 
 }