Find a way to exclude test data when building docker image.

I just noticed something. When trying to build a docker image with a clean-ish branch (no non-checked in files), I got this log trace from Google Cloud Build:
```
Creating temporary tarball archive of 159 file(s) totalling 190.3 MiB before compression.
```

I did an experiment: I deleted all the test data folders and re-rand the command to build our docker image:

```
Creating temporary tarball archive of 124 file(s) totalling 958.1 KiB before compression.
```

This is quite a huge improvement! In general, I wonder if there's a way where we can not have checked-in test data.