test(ci): pin RHDH 1.10 RC image (1.10-128) in showcase value files - #4903

Description

The new pinned upstream.backstage.image.{registry,repository,tag} values in the showcase value
files are overridden in CI because helm::install always appends --set upstream.backstage.image.*
from helm::get_image_params(). Since env_variables.sh assigns defaults for
IMAGE_REGISTRY/IMAGE_REPO, the pinned image can be silently replaced, causing release-1.10 tests
to run a different image than the value files specify.

Code

.ci/pipelines/value_files/values_showcase.yaml[R136-140]

Relevance

⭐ Low

Team treats --set/env as source of truth; PRs removed image pins from values to avoid drift.

PR-#4457
PR-#4463

ⓘ Recommendations generated based on similar findings in past PRs

Evidence

The values files set the pinned image, but the CI Helm path always passes `--set
upstream.backstage.image.*` from env-derived variables, which take precedence over value files.
Additionally, env_variables.sh provides defaults for IMAGE_REGISTRY/IMAGE_REPO, so the
override path is active even when jobs don't explicitly provide these vars.

.ci/pipelines/value_files/values_showcase.yaml[135-141]
.ci/pipelines/value_files/values_showcase-rbac.yaml[151-157]
.ci/pipelines/lib/helm.sh[218-229]
.ci/pipelines/lib/helm.sh[256-262]
.ci/pipelines/env_variables.sh[44-48]

Agent prompt

The issue below was found during a code review. Follow the provided context and guidance below and implement a solution

## Issue description
Showcase value files now pin `upstream.backstage.image.*`, but CI Helm installs always override these fields via `--set upstream.backstage.image.{registry,repository,tag}=...` from environment variables. Because `IMAGE_REGISTRY`/`IMAGE_REPO` are defaulted in `env_variables.sh`, the override happens even when jobs don't explicitly set image params, so the pin in the values files is effectively ignored.

## Issue Context
`helm::install` appends `$(helm::get_image_params)` unconditionally, and `helm::get_image_params` always emits all three `--set` flags. `env_variables.sh` defaults `IMAGE_REGISTRY` and `IMAGE_REPO`, making the override path always active.

## Fix Focus Areas
- .ci/pipelines/lib/helm.sh[218-262]
- .ci/pipelines/env_variables.sh[44-48]

### Concrete fix options (pick one)
1) **Add an explicit opt-out flag** (recommended):
  - Introduce e.g. `USE_VALUES_IMAGE=true` (or `SKIP_IMAGE_SET_FLAGS=true`).
  - In `helm::get_image_params`, return empty when the flag is set.
  - Set the flag in the specific release-1.10 jobs that should use the pinned values.

2) **Only emit `--set` flags when explicitly provided**:
  - Stop defaulting `IMAGE_REGISTRY`/`IMAGE_REPO` in `env_variables.sh` (or only default them when a separate `FORCE_IMAGE_PARAMS=true` is enabled).
  - In `helm::get_image_params`, only append `--set ...` for variables that are non-empty.

3) **If the intention is to override everywhere**, then remove the pin from the values files (since it is misleading) and instead set the desired defaults via env or job configuration.

ⓘ Copy this prompt and use it to remediate the issue with your preferred AI generation tools

• Pin RHDH 1.10 RC image (1.10-128) in showcase value files
• Update both showcase and showcase-rbac Helm value files
• Set explicit image registry, repository, and tag values
• Add clarifying comments about CI override behavior

Pin RHDH 1.10 RC image in showcase values

• Replace generic image comment with pinned RHDH 1.10 RC image reference
• Set registry: quay.io, repository: rhdh/rhdh-hub-rhel9, tag: "1.10-128"
• Add clarifying comment about potential CI override via --set flags

.ci/pipelines/value_files/values_showcase.yaml

Pin RHDH 1.10 RC image in showcase-rbac values

• Replace generic image comment with pinned RHDH 1.10 RC image reference
• Set registry: quay.io, repository: rhdh/rhdh-hub-rhel9, tag: "1.10-128"
• Add clarifying comment about potential CI override via --set flags

.ci/pipelines/value_files/values_showcase-rbac.yaml

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