Add devcontainer.json for Doppler engineers

This is intended for Doppler engineer use only, this image is not
publically accessible.

Author: emily-curry

.devcontainer/devcontainer.json

@@ -0,0 +1,4 @@
+{
+  "image": "us-central1-docker.pkg.dev/gar-prod-73b1cdf/devtools/devcontainer:latest",
+  "name": "cli"
+}