fix: set sh as default shell for containers (#853)

* fix: set default shell for containers to sh

This matches GitHub Actions behaviour where,
runners use bash since it's guaranteed to exist there
while containers use sh for compatibility

* tests: fixup for default shell

pkg/runner/testdata/shells/python/push.yml

@@ -7,3 +7,11 @@ jobs:
         run: |
           import platform
           print(platform.python_version())
+  check-container:
+    runs-on: ubuntu-latest
+    container: node:12-buster
+    steps:
+      - shell: python
+        run: |
+          import platform
+          print(platform.python_version())