[9.2] [Cases] Enable case feature flag overrides with ENV vars (#241271) (#241291)

# Backport

This will backport the following commits from `main` to `9.2`:
- [[Cases] Enable case feature flag overrides with ENV vars
(#241271)](#241271)

<!--- Backport version: 9.6.6 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

<!--BACKPORT [{"author":{"name":"Jan
Monschke","email":"[email protected]"},"sourceCommit":{"committedDate":"2025-10-30T14:12:57Z","message":"[Cases]
Enable case feature flag overrides with ENV vars (#241271)\n\n##
Summary\n\nAdding our feature flags to the env var override config. For
more info,\ncheck out [the documentation on env
var\noverrides](https://www.elastic.co/docs/deploy-manage/deploy/self-managed/install-kibana-with-docker#environment-variable-config).","sha":"5631baff8cb4ef35af3295a53be4b105695536aa","branchLabelMapping":{"^v9.3.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["release_note:skip","backport:version","Team:Cases","v9.3.0","v9.2.1"],"title":"[Cases]
Enable case feature flag overrides with ENV
vars","number":241271,"url":"https://github.com/elastic/kibana/pull/241271","mergeCommit":{"message":"[Cases]
Enable case feature flag overrides with ENV vars (#241271)\n\n##
Summary\n\nAdding our feature flags to the env var override config. For
more info,\ncheck out [the documentation on env
var\noverrides](https://www.elastic.co/docs/deploy-manage/deploy/self-managed/install-kibana-with-docker#environment-variable-config).","sha":"5631baff8cb4ef35af3295a53be4b105695536aa"}},"sourceBranch":"main","suggestedTargetBranches":["9.2"],"targetPullRequestStates":[{"branch":"main","label":"v9.3.0","branchLabelMappingKey":"^v9.3.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/241271","number":241271,"mergeCommit":{"message":"[Cases]
Enable case feature flag overrides with ENV vars (#241271)\n\n##
Summary\n\nAdding our feature flags to the env var override config. For
more info,\ncheck out [the documentation on env
var\noverrides](https://www.elastic.co/docs/deploy-manage/deploy/self-managed/install-kibana-with-docker#environment-variable-config).","sha":"5631baff8cb4ef35af3295a53be4b105695536aa"}},{"branch":"9.2","label":"v9.2.1","branchLabelMappingKey":"^v(\\d+).(\\d+).\\d+$","isSourceBranch":false,"state":"NOT_CREATED"}]}]
BACKPORT-->

Co-authored-by: Jan Monschke <[email protected]>

Author: kibanamachine

src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker

@@ -268,8 +268,14 @@ kibana_vars=(
     xpack.banners.placement
     xpack.banners.textColor
     xpack.banners.textContent
+    xpack.cases.enabled
+    xpack.cases.analytics.index.enabled
+    xpack.cases.incrementalId.enabled
+    xpack.cases.incrementalId.taskIntervalMinutes
+    xpack.cases.incrementalId.taskStartDelayMinutes
     xpack.cases.files.allowedMimeTypes
     xpack.cases.files.maxSize
+    xpack.cases.stack.enabled
     xpack.code.disk.thresholdEnabled
     xpack.code.disk.watermarkLow
     xpack.code.indexRepoFrequencyMs