add GHA extension to devcontainer

.devcontainer/devcontainer.json

@@ -35,6 +35,7 @@
                 "davidanson.vscode-markdownlint",
                 "ms-azuretools.vscode-docker",
                 "vscode-icons-team.vscode-icons",
+                "github.vscode-github-actions",
             ],
         }
     }