Exclude node_modules at all depths in Docker builds

The pattern `node_modules` only matches at the root level in
.dockerignore (unlike .gitignore). Use `**/node_modules` to
exclude client/node_modules and avoid copying 22GB to the
build context.

Author: dannon

.dockerignore

@@ -1,4 +1,4 @@
 .k8s_ci.Dockerfile
 .venv
 database
-node_modules
+**/node_modules