ci: Replace Travis with GitHub Actions

Adds GitHub Action for multi-architecture Docker image building and publishing, and a companion test script.

Thank you Travis CI for all the builds.  It is very appreciated
by myself and the community.

Built with AntiGravity and Claude Sonnet 4.5

```
this project builds docker images for openresty application server.  it uses scripts which invoke docker build commands which are driven by travis ci.   i want to migrate to github actions.
```

It proceeded to make an Action which used the scripts.  Thus I prompted:

```
is there a well supported github action which can achieve this?
```

i saw some issues by eyeball, so I prompted:

```
looking good.  can you double check your work in converting from the original build matrix structure and the new one
```

It fixed the issues I saw and more.

```
is there a way to test this locally?
```

It told me about `act` and gave instructions:

```
i've install act and am getting errors. can you run it and see if you can fix the errors?
```

It fixed the errors and I could test build.

Signed-off-by: Evan Wies <evan@neomantra.net>

Author: neomantra