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GH-43933: [CI] Remove docker-compose warnings (#43934)



### 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- `version` in `docker-compose.yml` is obsolete and should be removed.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: #43933

Authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>