fix: kaniko dockerfile path should be relative to context (#211)

Kaniko expects --dockerfile to be relative to --context, not an
absolute path. Changed from '/workspace/k8s/Dockerfile' to
'k8s/Dockerfile' since context is '/workspace'.

Reviewed-on: graham/towerops-web#211

.forgejo/workflows/production.yaml

@@ -122,7 +122,7 @@ jobs:
             -v /tmp/kaniko:/kaniko/.docker \
             gcr.io/kaniko-project/executor:latest \
             --context /workspace \
-            --dockerfile /workspace/k8s/Dockerfile \
+            --dockerfile k8s/Dockerfile \
             --destination ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${IMAGE_TAG} \
             --destination ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:production \
             --build-arg MIX_ENV=prod \