Upgrade Forgejo to v1.20.4-1
Also the URLs for the Forgejo files is now nicely formatted so the JSON
file for the Forgejo downloads is no longer needed and has been removed.
- Update the Woodpecker Dockerfile.
- Add .dockerignore to limit the context.
- Add code to download the Woodpecker binary.
- Refactor the prepare target.
- Refactor the code for the Download target to reduce repeated code
and (hopefully) improve readability.
- The code now checks to see if each of the downloaded files already
exists.
- The verification will always run if enabled regardless of whether the
files are already downloaded or not.
