ci: tag container images with dev alongside midstream

Add :dev tag to both gateway and cluster multi-arch manifests in the
midstream container build workflow. Local cargo builds default to the
dev tag (OPENSHELL_IMAGE_TAG is unset), so this ensures locally-built
CLI binaries can pull images from GHCR without needing to override
the tag. The dev and midstream tags are kept in sync — both point to
the same image built from the midstream branch on every merge.

Author: maxamillion

.github/workflows/midstream-container-build.yml

@@ -135,6 +135,7 @@ jobs:
           docker buildx imagetools create \
             -t "${{ env.IMAGE_REGISTRY }}/gateway:${{ github.sha }}" \
             -t "${{ env.IMAGE_REGISTRY }}/gateway:midstream" \
+            -t "${{ env.IMAGE_REGISTRY }}/gateway:dev" \
             "${{ env.IMAGE_REGISTRY }}/gateway:${{ github.sha }}-amd64" \
             "${{ env.IMAGE_REGISTRY }}/gateway:${{ github.sha }}-arm64"
 
@@ -159,5 +160,6 @@ jobs:
           docker buildx imagetools create \
             -t "${{ env.IMAGE_REGISTRY }}/cluster:${{ github.sha }}" \
             -t "${{ env.IMAGE_REGISTRY }}/cluster:midstream" \
+            -t "${{ env.IMAGE_REGISTRY }}/cluster:dev" \
             "${{ env.IMAGE_REGISTRY }}/cluster:${{ github.sha }}-amd64" \
             "${{ env.IMAGE_REGISTRY }}/cluster:${{ github.sha }}-arm64"