Add Renovate pipeline

[mthalman]

• Adds Renovate pipeline infrastructure for automated dependency updates
• Creates reusable job template for running Renovate against GitHub repositories
• Configures Renovate with custom regex manager for tracking version variables in Dockerfiles
• Updates src/Dockerfile.linux with Renovate tracking comment for ORAS version

The intent is for eng/pipelines/templates/jobs/renovate.yml and eng/renovate.env to be moved to Arcade once things have been evaluated in this repo first.

Related: dotnet/arcade#15594

[Copilot]

Pull request overview

This PR adds Renovate pipeline infrastructure to the dotnet/docker-tools repository for automated dependency updates. The implementation includes a reusable job template, configuration files, and a scheduled pipeline that will run nightly at 3 AM UTC.

Changes:

• Adds Renovate tracking comment to src/Dockerfile.linux for ORAS version monitoring
• Creates Renovate configuration with custom regex manager for Dockerfile ARG/ENV variables
• Implements reusable Azure Pipeline job template for running Renovate
• Sets up nightly scheduled pipeline with dry-run and force-recreate options

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/Dockerfile.linux	Adds Renovate tracking comment for ORAS_VERSION ARG to enable automated version updates
eng/renovate.json	Configures Renovate with custom regex manager and dockerfileVersions preset
eng/renovate.env	Defines environment variables for Renovate bot configuration including PR templates and author settings
eng/pipelines/templates/jobs/renovate.yml	Implements reusable job template that validates config, runs Renovate in Docker, and publishes logs
eng/pipelines/renovate.yml	Defines scheduled pipeline that runs nightly against dotnet/dotnet-docker repository

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