Merge upstream changes from Marlin 2.1.1

docker/Dockerfile

@@ -0,0 +1,7 @@
+FROM python:3.9.0-buster
+
+RUN pip install -U platformio
+RUN pio upgrade --dev
+# To get the test platforms
+RUN pip install PyYaml
+#ENV PATH /code/buildroot/bin/:/code/buildroot/tests/:${PATH}