`READONLY_CONFIG_FILE` doesn't work in docker containers

[MasterAwesome]

When READONLY_CONFIG_FILE is used, the file name passed to merge_config_files is the object as a string which is serialized and written into disk. This always results in a file named <tempfile._TemporaryFileWrapper object at 0x7fa12bf82350> on disk instead of the actual name and doesn't actually merge configs.

Bug is present in master branch of opengrok

To Reproduce

• Run the docker container built using the Dockerfile
• Set -e READONLY_CONFIG_FILE environment variable

Expected behavior Merging configs succeeds

Observed behavior Merging configs uses the object instead of the actual FS path <tempfile._TemporaryFileWrapper object at 0x7fa12bf82350> resulting in merge failure + read-only-config to not be applied.

[vladak]

This was introduced with the rework of how config merge is called (#4509).