ci: Fix - Secure github actions

The Docker login step was using username (NordicBuilder),
which caused `permission_denied: write_package` errors when switching
to GITHUB_TOKEN. Updated to use `${{ github.actor }}`, as required by
GHCR authentication with GITHUB_TOKEN.
Based on https://github.com/docker/login-action

Signed-off-by: Jakub Ciesla <[email protected]>

Author: jaci-nordic

.github/workflows/docker.yml

@@ -30,7 +30,7 @@ jobs:
         uses: docker/login-action@74a5d142397b4f367a81961eba4e8cd7edddf772 # v3
         with:
           registry: ghcr.io
-          username: NordicBuilder
+          username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Check if Docker image exists