fix: compile the right file

[joscha]

otherwise fails with:

g++: error: hello.c: No such file or directory
g++: fatal error: no input files
compilation terminated.
Removing intermediate container 5f48cf87fbe8
       Error while processing push: Error: The command '/bin/sh -c g++ -o hello hello.c' returned a non-zero code: 4

[shaunmulligan]

@josephroberts is this PR still valid?