Merge pull request #49388 from HotelsDotCom/feature/Dynamic-env-in-subpath

Automatic merge from submit-queue (batch tested with PRs 58920, 58327, 60577, 49388, 62306). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Dynamic env in subpath - Fixes Issue 48677

**What this PR does / why we need it**:

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #48677

**Special notes for your reviewer**:

**Release note**:

```release-note
Adds the VolumeSubpathEnvExpansion alpha feature to support environment variable expansion
Sub-paths cannot be mounted with a dynamic volume mount name.
This fix provides environment variable expansion to sub paths
This reduces the need to manage symbolic linking within sidecar init containers to achieve the same goal  
```

pkg/kubelet/container/helpers.go

@@ -133,6 +133,11 @@ func ExpandContainerCommandOnlyStatic(containerCommand []string, envs []v1.EnvVa
 	return command
 }
 
+func ExpandContainerVolumeMounts(mount v1.VolumeMount, envs []EnvVar) (expandedSubpath string) {
+	mapping := expansion.MappingFuncFor(EnvVarsToMap(envs))
+	return expansion.Expand(mount.SubPath, mapping)
+}
+
 func ExpandContainerCommandAndArgs(container *v1.Container, envs []EnvVar) (command []string, args []string) {
 	mapping := expansion.MappingFuncFor(EnvVarsToMap(envs))