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/container-hostname/push.yml

@@ -1,6 +1,10 @@
 name: container-hostname
 on: push
 
+defaults:
+  run:
+    shell: bash
+
 jobs:
   with-hostname:
     runs-on: ubuntu-latest