Add linux/arm64 to the docker build platforms.

[pixlwave]

There isn't an OCI container image published for ARM64, so this PR adds the platform to the docker workflow to make testing easier on macOS.

I've tested that it builds for arm64 with

container build --platform linux/arm64 --file docker/Dockerfile --tag matrix-content-scanner:arm64 .
container image inspect matrix-content-scanner:arm64

which was successful and indicates the correct platform

"platform" : {
  "architecture" : "arm64",
  "os" : "linux"
},

I can spin up a container from the image which starts and is reachable (but I haven't tried to scan any media).

Full disclosure: The changes were made with the suggestions from an LLM and they look correct to me from my research, but my personal experience in this area is basically limited to running container build xyz locally.