config/docker: add libubsan to kselftest fragment

[musamaanjum]

The libubsan is missing and is required for some kselftest suites such as openat2. The error is as following:

error while loading shared libraries: libubsan.so.1: cannot open shared object file: No such file or directory

[musamaanjum]

Request to review @nuclearcat @a-wai

[nuclearcat]

Tested on staging