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/sh/push.yml

@@ -5,6 +5,19 @@ jobs:
     steps:
       - shell: sh
         run: |
-          if [ -n "$PATH" ] ; then
+          if [ -z ${BASH+x} ]; then
             echo "I'm sh!"
+          else
+            exit 1
+          fi
+  check-container:
+    runs-on: ubuntu-latest
+    container: alpine:latest
+    steps:
+      - shell: sh
+        run: |
+          if [ -z ${BASH+x} ]; then
+            echo "I'm sh!"
+          else
+            exit 1
           fi