[installer]: add explanation for configuring AWS container registry

[mrsimonemms]

Description

Add a note of clarification on how to use AWS ECR as push registry. Thanks to @eiymba for asking the question

As was in this section, I've added some validation on the secret

Related Issue(s)

Fixes #8321

How to test

Release Notes

NONE

Documentation

[codecov]

Codecov Report

Merging #8322 (02c25f2) into main (f41c652) will decrease coverage by 25.40%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #8322       +/-   ##
==========================================
- Coverage   33.00%   7.60%   -25.41%     
==========================================
  Files          33      31        -2     
  Lines        4735    2171     -2564     
==========================================
- Hits         1563     165     -1398     
+ Misses       3054    2003     -1051     
+ Partials      118       3      -115     

Flag	Coverage Δ
components-gitpod-cli-app	11.17% <ø> (ø)
components-installer-raw-app	4.58% <ø> (?)
components-local-app-app-darwin-amd64	?
components-local-app-app-darwin-arm64	?
components-local-app-app-linux-amd64	?
components-local-app-app-linux-arm64	?
components-local-app-app-windows-386	?
components-local-app-app-windows-arm64	?
components-ws-manager-app	?

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Impacted Files	Coverage Δ
...s/ws-manager/pkg/manager/internal/grpcpool/pool.go
...omponents/ws-manager/pkg/manager/pod_controller.go
components/ws-manager/pkg/manager/create.go
components/ws-manager/pkg/manager/monitor.go
components/local-app/pkg/auth/pkce.go
components/ws-manager/pkg/manager/manager_ee.go
components/ws-manager/pkg/manager/metrics.go
...-manager/pkg/manager/internal/workpool/workpool.go
components/ws-manager/pkg/clock/clock.go
components/ws-manager/pkg/manager/status.go
... and 18 more

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