Improve devcontainer for running tests (#22277)

* Improve devcontainer for running tests

- Pull devcontainer post-create out into its own script
- Add asset precompilation
- Add test-mode asset precompilation (needed to run tests without error)

* Document Gemfile.lock re-checkout in devcontainer

.devcontainer/devcontainer.json

@@ -20,7 +20,7 @@
   "forwardPorts": [3000, 4000],
 
   // Use 'postCreateCommand' to run commands after the container is created.
-  "postCreateCommand": "bundle install --path vendor/bundle && yarn install && git checkout -- Gemfile.lock && ./bin/rails db:setup",
+  "postCreateCommand": ".devcontainer/post-create.sh",
 
   // Comment out to connect as root instead. More info: https://aka.ms/vscode-remote/containers/non-root.
   "remoteUser": "vscode"