Fix quickstart Dockerfile warnings

[Haihan-Jiang]

Description

This PR updates the remaining quickstart Dockerfiles that still use Dockerfile forms covered by the warnings in #1781. The main quickstarts/hello-app Dockerfile was already fixed in #1829; this follows the same format for related quickstart samples.

Changes:

• Use AS for multi-stage builder aliases.
• Use ENV key=value for quickstart environment variables.

Validation:

• git diff --check
• Checked the modified quickstart Dockerfiles no longer match FROM ... as or legacy ENV key value forms.
• Tried a local docker build for quickstarts/hello-app-cdn, but this machine is missing the Docker buildx component and the legacy builder stalled while pulling golang:1.24.3; leaving this as draft until CI/maintainers can confirm.

Related to #1781.

Tasks

• The contributing guide has been read and followed.
• The samples added / modified have been fully tested.
• Workflow files have been added / modified, if applicable.
• Region tags have been properly added, if new samples.
• Editable variables have been used, where applicable.
• All dependencies are set to up-to-date versions, as applicable.
• Merge this pull-request for me once it is approved.

[Haihan-Jiang]

This is ready for review from my side. The visible checks are green; could someone take a look when convenient?