fix!: fallback to sh if bash does not exist

It is a breaking change because it changes how the shell is
determined.

Before, if `jobs.<job_id>.container.image` is set and the shell is not
specified, it defaults to `sh` instead of `bash`.

After, regardless of `jobs.<job_id>.container.image`, if the shell is
not specified, it defaults to `bash` if available, otherwise it
defaults to `sh`.

Rework the shell integration tests:

- Remove container specific tests because the special behavior related
  to shell being set differently when a container image is present is
  removed
- Modify the defaults test case to verify the fallback logic
- Use container images from code.forgejo.org to escape rate limiting
  in the CI
- Add the missing node test
- Use
      container:
        image: code.forgejo.org/oci/node:22-bookworm
  instead of
      container: code.forgejo.org/oci/node:22-bookworm
  because it silently failed to run (with no exit code)
- Prefer `-z "${BASH}"` because `-z ${BASH+x}` reads obscure

Closes forgejo/runner#150

act/runner/step_run.go

@@ -194,9 +194,20 @@ func (sr *stepRun) setupShell(ctx context.Context) {
 			if err != nil {
 				step.Shell = shellWithFallback[1]
 			}
-		} else if containerImage := rc.containerImage(ctx); containerImage != "" {
-			// Currently only linux containers are supported, use sh by default like actions/runner
-			step.Shell = "sh"
+		} else {
+			shell_fallback := `
+if command -v bash >/dev/null; then
+	echo -n bash
+else
+	echo -n sh
+fi
+`
+			stdout, _, err := rc.sh(ctx, shell_fallback)
+			if err != nil {
+				common.Logger(ctx).Error("fail to run %q: %v", shell_fallback, err)
+				return
+			}
+			step.Shell = stdout
 		}
 	}
 }