fix: consistently use absolute paths for the binary containers

[codymlewis]

Issue

When attempting to run the PoC submission server in binary only mode, I would get the following error:

Client Error for http+docker://localhost/v1.55/containers/create: Bad Request ("create cybergym-server-data/arvo/10400/vul/arvo: "cybergym-server-data/arvo/10400/vul/arvo" includes invalid characters for a local volume name, only "[a-zA-Z0-9][a-zA-Z0-9_.-]" are allowed.

This comes from the inconstant use of absolute paths in the run_container_binary function in src/cybergym/server/server_utils.py.

Changes

Now all paths used in container volumes in the run_container_binary function are converted to absolute paths before conversion to strings.

[wzunknown]

what about ensuring the data_dir is absoluate path at the beginning?

data_dir = data_dir.absolute()

[codymlewis]

thanks, that's much cleaner. I have made both data_dir and poc_path absolute at the beginning now, and removed the other absolute calls where unnecessary.