Use the Dockerfile COPY instead of docker cp (#10943)

## Problem
We use `docker cp` to copy the files required for the extension tests
now.
It causes problems if we run older images with the newer source tree.
## Summary of changes
Copying the files was moved to the compute Dockerfile.

.dockerignore

@@ -14,6 +14,7 @@
 !compute/
 !compute_tools/
 !control_plane/
+!docker-compose/ext-src
 !libs/
 !pageserver/
 !pgxn/