[openjdk] explicitly set entrypoint to bash

this prevents the containers from launching jshell potentially interfering with
the tests, see [poo#174976](https://progress.opensuse.org/issues/174976).

tests/test_openjdk_devel.py

@@ -27,6 +27,7 @@
 DOCKERF_EXTENDED = f"""
 WORKDIR {CONTAINER_TEST_DIR}
 COPY {HOST_TEST_DIR} {CONTAINER_TEST_DIR}
+ENTRYPOINT ["/bin/bash"]
 """
 
 CONTAINER_IMAGES_EXTENDED = [