Cirrus: Use VM instead of Docker for functional tests

.cirrus.yml

@@ -88,13 +88,15 @@ task:
 
 task:
   name: Regtest functional tests
-  container:
-    image: $ELECTRUM_IMAGE
+  compute_engine_instance:
+    image_project: cirrus-images
+    image: family/docker-builder
+    platform: linux
     cpu: 1
     memory: 1G
   pip_cache:
     folder: ~/.cache/pip
-    fingerprint_script: echo Regtest && echo $ELECTRUM_IMAGE && cat $ELECTRUM_REQUIREMENTS
+    fingerprint_script: echo Regtest && echo docker_builder && cat $ELECTRUM_REQUIREMENTS
     populate_script: mkdir -p ~/.cache/pip
   electrum_cache:
     folder: /tmp/electrum-build
@@ -121,7 +123,6 @@ task:
     - sleep 10s
     - python3 -m unittest electrum/tests/regtest.py
   env:
-    ELECTRUM_IMAGE: python:3.7
     ELECTRUM_REQUIREMENTS: contrib/requirements/requirements-travis.txt
     # ElectrumX exits with an error without this:
     ALLOW_ROOT: 1