Update mirror.gcr.io/library/ubuntu Docker tag to v26 - autoclosed

[renovate-bot]

This PR contains the following updates:

Package	Type	Update	Change
mirror.gcr.io/library/ubuntu	final	major	24.04 → 26.04

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[gemini-code-assist]

Code Review

This pull request updates the base image in the Dockerfile from Ubuntu 24.04 to Ubuntu 26.04. However, this upgrade breaks the Docker build because Ubuntu 26.04 completely removes apt-key, which is still used in several commands within the Dockerfile. These commands must be migrated to use /etc/apt/keyrings/ before upgrading the base image.

[gemini-code-assist]

Upgrading to Ubuntu 26.04 breaks the Docker build because apt-key has been completely removed in Ubuntu 26.04. Several commands in this Dockerfile still rely on apt-key add (such as lines 74, 102, and 253). Before upgrading the base image to 26.04, these steps must be migrated to use the modern approach of downloading keys to /etc/apt/keyrings/ and referencing them via the signed-by option in the repository sources files.