chore: reduce docker image size (#194)

This PR makes changes in the Dockerfile to reduce the final image size.
I'm sure this can be optimized further. But I didn't want to make too
many changes at once.

Original: 
```
REPOSITORY   TAG       IMAGE ID       CREATED         SIZE
codex        latest    c8f66942ded3   4 minutes ago   1.91GB
```

Now: 
```
REPOSITORY   TAG       IMAGE ID       CREATED         SIZE
codex        latest    2a6d6450609c   4 minutes ago   747MB
```