regtests: mitigate file permissions issue with podman compose

This change works around a file-permissions issue when running regtests via `podman compose up --build --exit-code-from regtest` (not Docker). CI is happy without this change.

Generally speaking, a `chmod -R go+rwx` is not good, however, it is only for tests running in a container.

```
[regtest] | t_cli/src/test_cli.py ....                                               [ 94%]
[regtest] | t_pyspark/src/test_spark_sql_s3_with_privileges.py ssssssssssss          [100%]
[regtest] |
[regtest] | =============================== warnings summary ===============================
[regtest] | ../polaris-venv/lib/python3.8/site-packages/_pytest/cacheprovider.py:479
[regtest] |   /home/spark/polaris-venv/lib/python3.8/site-packages/_pytest/cacheprovider.py:479: PytestCacheWarning: could not create cache path /home/spark/regtests/.pytest_cache/v/cache/nodeids: [Errno 13] Permission denied: '/home/spark/regtests/pytest-cache-files-nm7of75m'
[regtest] |     config.cache.set("cache/nodeids", sorted(self.cached_nodeids))
[regtest] |
[regtest] | ../polaris-venv/lib/python3.8/site-packages/_pytest/stepwise.py:57
[regtest] |   /home/spark/polaris-venv/lib/python3.8/site-packages/_pytest/stepwise.py:57: PytestCacheWarning: could not create cache path /home/spark/regtests/.pytest_cache/v/cache/stepwise: [Errno 13] Permission denied: '/home/spark/regtests/pytest-cache-files-gxv4jemz'
[regtest] |     session.config.cache.set(STEPWISE_CACHE_DIR, [])
[regtest] |
[regtest] | -- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
[regtest] | =========== 218 passed, 12 skipped, 2 warnings in 139.01s (0:02:19) ============
[regtest] | Wed 21 Aug 2024 11:21:46 AM UTC: Test SUCCEEDED: t_cli:test_cli.py
[regtest] | Wed 21 Aug 2024 11:21:46 AM UTC: Starting test t_hello_world:hello_world.sh
mkdir: cannot create directory ‘/tmp/polaris-regtests/t_hello_world’: Permission denied
/home/spark/regtests/run.sh: line 121: /tmp/polaris-regtests/t_hello_world/hello_world.sh.stderr: No such file or directory
/home/spark/regtests/run.sh: line 121: /tmp/polaris-regtests/t_hello_world/hello_world.sh.stdout: No such file or directory
[regtest] | Wed 21 Aug 2024 11:21:46 AM UTC: Test run concluded for t_hello_world:hello_world.sh
/home/spark/regtests/run.sh: line 132: /tmp/polaris-regtests/t_hello_world/hello_world.sh.fixdiffs.sh: No such file or directory
[regtest] | Wed 21 Aug 2024 11:21:46 AM UTC: Test FAILED: t_hello_world:hello_world.sh
/home/spark/regtests/run.sh: line 133: /tmp/polaris-regtests/t_hello_world/hello_world.sh.fixdiffs.sh: No such file or directory
chmod: cannot access '/tmp/polaris-regtests/t_hello_world/hello_world.sh.fixdiffs.sh': No such file or directory
[regtest] | Wed 21 Aug 2024 11:21:46 AM UTC: To compare and fix diffs (if 'meld' installed): /tmp/polaris-regtests/t_hello_world/hello_world.sh.fixdiffs.sh
[regtest] | Wed 21 Aug 2024 11:21:46 AM UTC: Or manually diff: diff /tmp/polaris-regtests/t_hello_world/hello_world.sh.stdout /home/spark/regtests/t_hello_world/ref/hello_world.sh.ref
[regtest] | Wed 21 Aug 2024 11:21:46 AM UTC: See stderr from test run for additional diagnostics: /tmp/polaris-regtests/t_hello_world/hello_world.sh.stderr
diff: /tmp/polaris-regtests/t_hello_world/hello_world.sh.stdout: No such file or directory
[regtest] | Wed 21 Aug 2024 11:21:46 AM UTC: Starting pytest t_pyspark:test_spark_sql_s3_with_privileges.py
[regtest] | ============================= test session starts ==============================
[regtest] | platform linux -- Python 3.8.10, pytest-8.2.2, pluggy-1.5.0
[regtest] | rootdir: /home/spark/regtests
```

regtests/Dockerfile

@@ -49,4 +49,7 @@ RUN ./setup.sh
 
 COPY --chown=spark . /home/spark/regtests
 
-CMD ["./run.sh"]
+# /home/spark/regtests might not be writable in all situations, see https://github.com/apache/polaris/pull/205
+RUN chmod -R go+rwx /home/spark/regtests
+
+CMD ["./run.sh"]