Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [x] Make sure to open an issue as a [bug/issue](https://togithub.com/googleapis/java-bigquerystorage/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)

In this PR:
- Remove generated files after running tests.

Note that files generated by `docker run` have root permission, thus
can't be removed by this PR. This issue will be resolved in #2615.

#2615)

## Context
[Milestone 7 of Hermetic
Build](https://docs.google.com/document/d/1v-sJBmdNEVBRryL8n90XK8OIiqVphaJemXVhANw14Jk/edit?pli=1&resourcekey=0-QGUb2do-JBlDWKvptWBp_g&tab=t.0#bookmark=id.bkbj04ib2d4n)

## In this PR
- Bake synthtool into the docker image (we won't `git clone` it anymore)
- Bake the owlbot CLI into the docker image (we won't use docker inside
docker anymore)
- Create a DEVELOPMENT.md file with instructions for local setup
(example: install owlbot CLI locally)

## Follow ups
- [ ] modify google-cloud-java's workflows to correct the volume mapping
being used

---------

Co-authored-by: Joe Wang <[email protected]>

…o v1.143.1 (#2615)

…o v1.143.1 (#2615)