Build image for `linux/arm64`

[brokeyourbike]

• specifying platforms in the build step
• updated actions to the latest versions

fixes #268

[brokeyourbike]

@hibiken can you please take a look if this is good enough?

[sruehl]

@brokeyourbike one note: I think that needs to be adjusted too, right? ENV CGO_ENABLED=0 GOOS=linux GOARCH=amd64 https://github.com/hibiken/asynqmon/blob/master/Dockerfile#L42