Build multi-platform Docker images

[maquchizi]

• This will build and push images for linux/amd64 and linux/arm64 to Docker Hub
• Necessitated by the fact that some of our infra is ARM-based

Context: When trying to deploy the latest changes to prod, I encountered the following error:

 1 warning found (use docker --debug to expand):
- InvalidBaseImagePlatform: Base image codeforafrica/pesacheck-ghost:v6.12.0 was pulled with platform "linux/amd64", expected "linux/arm64" for current build (line 1)
-----> Releasing pesacheck...
-----> Checking for predeploy task
     No predeploy task found, skipping
!     Detected linux/amd64 image, forcing --platform=linux/amd64
#0 building with "default" instance using docker driver

It turns out that our host system is actually ARM and therefore we need to build and push the appropriate image for that architecture.

[kilemensi]

LGTM!

---

Last I tried multi-platform build, they were so slow that I just decided to make sure both DEV and PROD use the same architecture (arm64 wherever possible). If things haven't improved, we might have to do the same for ghost (if we haven't).

[kilemensi]

Why are we pinning to a specific commit version instead of a specific release e.g.v3 or v3.0.7 if we care about being exact down to a fix?

[maquchizi]

I'm following Github's security recommendation for action pinning. I've pinned all actions to their latest release.

I know it's uglier that the usual v6 but the good news is Dependabot understands this format and can still keep our actions updated.

[maquchizi]

@kilemensi Fingers crossed that things have improved in that regard. If not, we can revert this change and only build for arm64 since that's what we're using.