style(executor_docker): use f-string for docker ps --format value

Replaces a string-concatenation expression with an f-string so the
Go-template format string can be read in one go. The doubled braces
(``{{{{`` / ``}}}}``) escape to literal ``{{`` / ``}}`` inside the
f-string, matching Docker's template syntax. Output is byte-identical
to the previous concatenation.

Addresses review feedback on PR #31.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Danelegend

code-interpreter/app/services/executor_docker.py

@@ -462,7 +462,7 @@ def reap_expired_sessions(self) -> int:
             "--filter",
             f"label=component={SESSION_COMPONENT_LABEL}",
             "--format",
-            '{{.Names}}\t{{.Label "' + SESSION_EXPIRES_AT_KEY + '"}}',
+            f'{{{{.Names}}}}\t{{{{.Label "{SESSION_EXPIRES_AT_KEY}"}}}}',
         ]
         try:
             list_result = subprocess.run(  # nosec B603