Proposed title of this feature request
image-registry should store/handle `REGISTRY_HTTP_SECRET` differently (in a secret)

What is the nature and description of the request?
There is security concerns (ROX-19897) about how the REGISTRY_HTTP_SECRET is currently handle(env. var) in the image-registry. Customer security teams would like to have this value moved into a secret instead of environment variable.

Who is the customer behind the request?
Account name: AXA Group Operations SAS
Account number: 1054801
TAM customer: yes
CSM customer: no
Strategic: no

List any affected packages or components:
image-registry