java-openliberty: add stack image label

[awisniew90]

Checklist:

• [ x] Read the Code of Conduct and Contributing Guidelines.

• [ x] Followed the commit message guidelines.

• [ x] Stack adheres to Appsody stack structure.

Modifying an existing stack:

• [x ] Updated the stack version in stack.yaml

Add image labels to Dockerfile-stack image

[awisniew90]

@neeraj-laad - updated the labels with stack variables like you suggested. Keeps things cleaner.