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
```