Fix registry secret names and add busybox to SDK

Use REGISTRY_USERNAME/REGISTRY_PASSWORD instead of
DOCKERHUB_SVRNTY_USERNAME/DOCKERHUB_SVRNTY_ACCESS_TOKEN.
Add busybox to SDK apko config for /bin/sh support.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitea/workflows/build.yaml

@@ -91,8 +91,8 @@ jobs:
       - name: Log in to DockerHub
         uses: docker/login-action@v3
         with:
-          username: ${{ secrets.DOCKERHUB_SVRNTY_USERNAME }}
-          password: ${{ secrets.DOCKERHUB_SVRNTY_ACCESS_TOKEN }}
+          username: ${{ secrets.REGISTRY_USERNAME }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
 
       - name: Build apko base images
         run: |

.gitea/workflows/scout.yaml

@@ -20,8 +20,8 @@ jobs:
       - name: Log in to DockerHub
         uses: docker/login-action@v3
         with:
-          username: ${{ secrets.DOCKERHUB_SVRNTY_USERNAME }}
-          password: ${{ secrets.DOCKERHUB_SVRNTY_ACCESS_TOKEN }}
+          username: ${{ secrets.REGISTRY_USERNAME }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
 
       - name: Check if latest runtime-lts image exists
         id: check
@@ -57,8 +57,8 @@ jobs:
       - name: Log in to DockerHub
         uses: docker/login-action@v3
         with:
-          username: ${{ secrets.DOCKERHUB_SVRNTY_USERNAME }}
-          password: ${{ secrets.DOCKERHUB_SVRNTY_ACCESS_TOKEN }}
+          username: ${{ secrets.REGISTRY_USERNAME }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
 
       - name: Discover latest LTS version
         id: discover

.gitea/workflows/update-check.yaml

@@ -19,8 +19,8 @@ jobs:
       - name: Log in to DockerHub
         uses: docker/login-action@v3
         with:
-          username: ${{ secrets.DOCKERHUB_SVRNTY_USERNAME }}
-          password: ${{ secrets.DOCKERHUB_SVRNTY_ACCESS_TOKEN }}
+          username: ${{ secrets.REGISTRY_USERNAME }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
 
       - name: Check for new .NET versions
         id: check