Mount an emptyDir for the Next.js cache in rallly (5.0.0+up4.12.3)

The chart mounted no volumes at all while running with
readOnlyRootFilesystem: true, so every Next.js write to
/app/apps/web/.next/cache failed. The image optimizer's failure even
surfaced as an unhandledRejection, so this was more than log noise.

The cache path is now a writable emptyDir; path and an optional
sizeLimit are configurable under rallly.cache. The hardening is
untouched — the pod's existing fsGroup (1000) is what makes the mounted
volume writable for uid 20000.

Verified in a throwaway k3d cluster which paths the app actually writes:
with a fully writable root filesystem and the cache mounted, the only
path the container touches is the cache directory itself. /tmp, $HOME
(=/app) and the rest of the root filesystem stay untouched, so no second
mount is needed.

Also drops the vestigial "capabilities.add: []": Trivy's AVD-KSV-0022 for
rallly (#84) comes from the older chart version deployed in the cluster,
which rendered a null list entry under add:. Reproduced with trivy config
against both renderings; the app needs no capabilities.

Major bump: runtime behavior changes, a volume is added.

Fixes #60

rallly/Chart.yaml

@@ -5,7 +5,7 @@ icon: https://raw.githubusercontent.com/lukevella/rallly/main/apps/landing/publi
 
 type: application
 
-version: 4.0.0+up4.12.3
+version: 5.0.0+up4.12.3
 appVersion: "4.12.3"
 
 maintainers: