Build multi-architecture images

[4141done]

What

Allows us to push multi-architecture builds for linux/amd64 and linux/arm64 on merge to master. Fixes: https://github.com/nginxinc/nginx-s3-gateway/issues/196

The flow looks like this:

                                   ┌──────────────────┐        ┌────────────────┐      ┌────────────────┐
    ┌─────────┐     ┌─────────┬────► Build Latest NJS ├────────►Test Latest NJS ├─────►│Push Latest NJS │
    │Build OSS├────►│Test OSS │    └──────────────────┘        └────────────────┘      └────────────────┘
    └─────────┘     └──┬──────┤
                       │      │    ┌──────────────────┐       ┌──────────────────┐     ┌─────────────────┐
                       │      └────►Build Unprivileged├───────►Test Unprivileged ├────►│Push Unprivileged│
                       │           └──────────────────┘       └──────────────────┘     ├────────┬────────┘
                       │                                                               ├────────┤
                       └──────────────────────────────────────────────────────────────►│Push OSS│
                                                                                       └────────┘

Tests are performed by building the relevant image and tagging it with the image name the tests expect. This follows the original implementation in test.sh. This will likely change to an explicit image name specification as we move the test suite in to Javascript but it's being maintained for now in the interest of simplicity.

Goals

• To decouple image build from the test run. This will help as we refactor the test suite for this issue https://github.com/nginxinc/nginx-s3-gateway/issues/191.
• Not have CI take a long time due to multi-architecture builds for interim pushes to pull requests.
• CI should fail fast if any of the image builds or tests fail
• Parallelize builds as much as possible.

Notes

• Since the unprivileged and latest-njs images build off the base oss image, there were some strange moves that had to be done to make that work without rebuilding the base image every time. Specifically, we had to set setup-buildx-action to run in docker driver mode so we can simply load the base image. Otherwise we would have had to use a local repository. However, in docker mode upload-artifact doesn't like the file produced so we have to save the file again.
• I wanted to build images once and then run tests against them and push at the end. However, there was not a clean way to get the full multi-architecture images all the way to the push step so I just build them again against all architectures and perform tests in the runner architecture. This saves us from needing conditionals in the test portions.