Remove airflow.utils dependency from Docker provider

[shaealh]

Why

Issue #62063 tracks removing provider dependencies on airflow.utils as part of the provider/client separation work. The Docker provider had one remaining dependency in DockerSwarmOperator for generating temporary service names.

What changed

• Replaced the airflow.utils.strings.get_random_string import with a private provider-local helper.
• Preserved the existing service-name format: the configured prefix followed by eight alphanumeric characters.
• Added focused unit coverage for the generated service name.
• Confirmed that providers/docker/src no longer imports airflow.utils.

This does not introduce any user-facing behavior changes.

Testing

• Commit and pre-commit checks passed.
• Docker Swarm operator tests: 25 passed.
• Full Docker provider suite attempted: 143 passed and 2 skipped; 15 Docker-dependent tests could not run because a local Docker daemon was unavailable.

related: #62063

---