Testing this process showed that multi-platform builds can be quite flaky or extremely
slow, depending on the complexities of each image's Dockerfile etc. With this in mind,
this commit leaves the default image type as `linux/amd64` but allows us to override
each build job as required using the `IMAGE_ARCHS` variable.

Initially, all `logan-terraform` jobs have been set to build both amd64 and arm64 image
types as this is the main requirement at this time.

Given the differences between the `docker build` and `docker buildx build` command usage,
the previous job functions have been refactored as a standalone `build.sh` file. The logic
is identical to the previous functions, however, the method is slightly different. This is
mostly due to the `docker buildx` command not having `tag` sub-command, instead, it uses
`--tag` and `--push` options on a single `docker buildx build` command to both build and
push images in one go.