Update the script for testing in docker container:

- don't require bash
  - mark appimage stage
  - use --no-cache if docker build fails
  - remove container after running the tests
  - use tty for colorful output

run-tests-in-docker.sh

@@ -1,5 +1,13 @@
-#!/bin/bash
+#!/bin/sh
 
-docker build --target travis -t pybm -f packages/docker/Dockerfile.bionic .
+DOCKERFILE=packages/docker/Dockerfile.bionic
-docker run pybm
 
+# explicitly mark appimage stage because it builds in any case
+docker build --target appimage -t pybm/appimage -f $DOCKERFILE .
+
+if [ $? -gt 0 ]; then
+    docker build --no-cache --target appimage -t pybm/appimage -f $DOCKERFILE .
+fi
+
+docker build --target tox -t pybm/tox -f $DOCKERFILE .
+docker run --rm -t pybm/tox