fix(docker-compose): escape $ in airflow-init to properly set AIRFLOW_UID

[jjunier]

Problem Description
In the docker-compose.yaml file, the airflow-init service contains an incorrect variable expansion export AIRFLOW_UID=$(id -u) which gets interpreted by Docker Compose as a template variable instead of being executed as a bash command inside the container.

Solution
Changed export AIRFLOW_UID=$(id -u) to export AIRFLOW_UID=$$(id -u) to properly escape the dollar sign, ensuring the command is executed inside the container rather than being interpreted by Docker Compose during template processing.

Changes Made

• Fixed variable escaping in docs/apache-airflow/howto/docker-compose/docker-compose.yaml
• The change maintains consistency with other bash command escaping patterns used throughout the same file (e.g., mem_available=$$(...), cpus_available=$$(...), disk_available=$$(...))

Testing

• Validated docker-compose.yaml syntax with docker-compose -f docker-compose.yaml config
• Confirmed the change aligns with existing patterns in the file where $$ escaping is used for bash commands

closes: #52959

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[boring-cyborg]

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[github-actions]

Backport successfully created: v3-0-test

Status	Branch	Result
✅	v3-0-test