Remove unused DOCKER_REPO variable causing pipeline failures

[Copilot]

The recent infrastructure update from PR #1490 introduced a pipeline failure where the stagingRepoPrefix variable was referenced but not defined. The error occurred in the Define ImageBuilder Command Variables step:

stagingRepoPrefix: /mnt/vss/_work/_temp/1a374083-69b0-4f7c-a0ae-80ebc6427d5c.ps1:14
Line |
  14 |  … DOCKER_REPO=msdotnetcontainers.azurecr.io/$(stagingRepoPrefix)dotnet- …
     |                                                ~~~~~~~~~~~~~~~~~
     | The term 'stagingRepoPrefix' is not recognized as a name of a cmdlet,
     | function, script file, or executable program.

The issue was caused by the DOCKER_REPO environment variable in eng/pipelines/variables/common.yml referencing $(stagingRepoPrefix) as a variable, but this value was only defined as a parameter in publishing config templates with no corresponding variable definition.

After analysis, it was determined that the DOCKER_REPO environment variable is not used anywhere else in the codebase and can be safely removed. This change removes the problematic variable definition entirely, resolving the pipeline failure with minimal impact.

Changes made:

• Removed the unused build.imageBuilderDockerRunExtraOptions variable that set DOCKER_REPO
• Confirmed no other references to DOCKER_REPO exist in the repository
• Validated YAML syntax remains correct

Fixes #1494.

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