[Bug]: Kubernetes: `registry_auth` ignored

[un-def]

Steps to reproduce

# .dstack.yml
type: task
name: demo
image: registry.example.com/demo
registry_auth:
  username: ${{ secrets.REGISTRY_USERNAME }}
  password: ${{ secrets.REGISTRY_PASSWORD }}

dstack apply --backend kubernetes

Actual behaviour

kubectl events --types Warning dstack-main-demo-0-0-s5urj0f4
LAST SEEN           TYPE      REASON   OBJECT                              MESSAGE
20s (x3 over 61s)   Warning   Failed   Pod/dstack-main-demo-0-0-s5urj0f4   Failed to pull image "registry.example.com/demo": failed to pull and unpack image "registry.example.com/demo:latest": failed to resolve reference "registry.example.com/demo:latest": pull access denied, repository does not exist or may require authorization: authorization failed: no basic auth credentials
20s (x3 over 61s)   Warning   Failed   Pod/dstack-main-demo-0-0-s5urj0f4   Error: ErrImagePull
6s (x3 over 61s)    Warning   Failed   Pod/dstack-main-demo-0-0-s5urj0f4   Error: ImagePullBackOff

Expected behaviour

No response

dstack version

0.20.17

Server logs

Additional information

No response