Default to Bazel target architecture

# 🚀 feature request

### Relevant Rules

container_image

### Description

The target architecture is always transitioned. By default it transitions to amd64.

The sensible behavior is to use the current target architecture. It's weird that it doesn't, and rules_docker seems to have gone out of its way(?) to make it like that.

### Describe the solution you'd like

--

### Describe alternatives you've considered

Disable rules_docker transitions and do it myself.