GH-43933: [CI] Remove docker-compose warnings

[lysnikolaou]

Rationale for this change

Remove docker-compose warnings to avoid confusion when running docker-compose and see them appear at the top. The two warnings are about:

• PYTHON_IMAGE_TAG not having a default value in .env
• version in docker-compose.yml is obsolete and should be removed.

What changes are included in this PR?

• Add a PYTHON_IMAGE_TAG default value in .env
• Remove version from docker-compose.yml

Are these changes tested?

Yes, the warnings are removed with these changes.

Are there any user-facing changes?

No.

• GitHub Issue: Docker warnings about docker-compose version and PYTHON_IMAGE_TAG default value #43933

[github-actions]

⚠️ GitHub issue #43933 has been automatically assigned in GitHub to PR creator.

[raulcd]

@github-actions crossbow submit test-cuda-*

[raulcd]

Thanks for the PR!
The only runner images that haven't been updated to the latest docker-compose versions are the cuda runners (there is currently some effort to do so). I am just validating the jobs doesn't fail for those when removing the version from the docker-compose.yml file. If the jobs are successful I am happy to merge!

[github-actions]

Revision: 7ff037a

Submitted crossbow builds: ursacomputing/crossbow @ actions-932da7be5a

Task	Status
test-cuda-cpp
test-cuda-python

[raulcd]

Thanks!

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit c455d6b.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 6 possible false positives for unstable benchmarks that are known to sometimes produce them.