Host-built sample challenge not guaranteed to work in challenge container

In the [local testing walkthrough](https://google.github.io/kctf/local-testing.html) there's an instruction to build the challenge binary using the host toolchain:

```
make -C challenge
```

However, my Ubuntu 22.04 host builds a binary with a different glibc version requirement and this results in the challenge not running in the container:

```
[I][2022-08-20T20:44:18+0000] Executing '/home/user/chal' for '[STANDALONE MODE]'
/home/user/chal: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by /home/user/chal)
```