Keep bufbuild-go-gen image under opiproject organization repo

[artek-koltun]

see #404 (comment)

[sandersms]

@glimchb Do we have a opiproject repo where we can place any tools that we have to create for version control and updates?

[glimchb]

@sandersms keep it here in this repo under https://github.com/opiproject/opi-api like we do with all images

for example

$ docker pull ghcr.io/opiproject/opi-api:v0.1.1

we can write a separate action to build and push it on changes in the docker file

like we do in other repos...