fix(helm): MEMORY_LIMIT env for custom controller container name

[flavono123]

Fixes #7650

Description

the value controller.containerName is supported #7384, but not for relevant env set by downward api.

1. fix the env MEMORY_LIMIT to refer controller.containerName if set
2. pull up both container name and env to the helper function

How was this change tested?

❯ helm template charts/karpenter --set controller.containerName="custom-name" 2>/dev/null | yq '...comments="" | select(.kind=="Deployment").spec.template.spec.containers[0].env[] | select(.name=="MEMORY_LIMIT")' 
name: MEMORY_LIMIT
valueFrom:
  resourceFieldRef:
    containerName: custom-name
    divisor: "0"
    resource: limits.memory

❯ helm template charts/karpenter --set controller.containerName="custom-name" 2>/dev/null | yq '...comments="" | select(.kind=="Deployment").spec.template.spec.containers[0].name' 
custom-name

Does this change impact docs?

• Yes, PR includes docs updates
• Yes, issue opened: #
• No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[netlify]

✅ Deploy Preview for karpenter-docs-prod canceled.

Name	Link
🔨 Latest commit	e6bc1b4
🔍 Latest deploy log	https://app.netlify.com/sites/karpenter-docs-prod/deploys/67a5a8ac9570bb000884f4a7

[jonathan-innis]

LGTM 🚀

[coveralls]

Pull Request Test Coverage Report for Build 13194524531

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 64.926%

---

Files with Coverage Reduction	New Missed Lines	%
pkg/fake/utils.go	2	94.81%

Totals
Change from base Build 13191304092:	-0.02%
Covered Lines:	5805
Relevant Lines:	8941

---

💛 - Coveralls

[jonathan-innis]

/karpenter snapshot