[branch-4.1][fix](docker) initialize external MS argument

[hello-stephen]

What problem does this PR solve?

TeamCity S3 Docker build #203806 reran branch-4.1 after #64623 and exposed the next first error shared by all 192 failed suites:

NameError: name 'external_ms_cluster' is not defined

This is a partial-backport gap. branch-4.1 already contains the --external-ms argument and passes external_ms_cluster to Cluster.new, but missed the corresponding local-variable assignment from #57236.

Jira: DORIS-26392
Build: http://172.20.48.17:8111/build/203806
Reference: #57236

What is changed?

Initialize external_ms_cluster from args.external_ms before the cloud/non-cloud branch.

This PR intentionally does not backport the full multi-cluster feature or add new validation behavior. It only restores the missing assignment while preserving the existing branch-4.1 behavior.

Check List

• Diff against branch-4.1 is exactly one added line.
• git diff --check
• python3 -m py_compile docker/runtime/doris-compose/command.py docker/runtime/doris-compose/cluster.py
• AST check confirms the variable is bound before Cluster.new uses it.
• After merge, rerun branch-4.1 Compile and S3 Docker; all #203806 occurrences must create their clusters and reach their own assertions.

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[wyxxxcat]

pick #57236 related modify for external ms cluster

[wyxxxcat]

LGTM

[github-actions]

PR approved by anyone and no changes requested.

[hello-stephen]

skip buildall