ci: point the image builds at the real Dockerfile locations

[rennerdo30]

Second failure on main, uncovered by #98. With Backend Build and Frontend Quality green again, the Docker Build job ran for the first time in months — and failed instantly:

ERROR: failed to build: failed to solve: failed to read dockerfile:
open Dockerfile.kernel: no such file or directory

The workflow pointed at deploy/compose/Dockerfile.kernel and deploy/compose/Dockerfile.worker, but the repository only has:

deploy/docker/Dockerfile.kernel
deploy/docker/Dockerfile.ui
deploy/docker/Dockerfile.worker

deploy/compose/docker-compose.yml already uses the correct deploy/docker/... paths, so the workflow was simply out of step.

Why it stayed hidden

Docker Build is gated on github.event_name == 'push' && github.ref == 'refs/heads/main' and needs backend-build and frontend-quality. It is skipped on every pull request, and it was skipped on main too for as long as Frontend Quality was failing. It only became reachable once #98 landed.

Verification

The job cannot be exercised from a PR, so both images were built locally with the corrected paths:

docker build -f deploy/docker/Dockerfile.kernel -t webencode-kernel-test .   # succeeds
docker build -f deploy/docker/Dockerfile.worker -t webencode-worker-test .   # succeeds