`container` automatically enables Rosetta for amd64 containers. This
change allows Rosetta to be enabled for arm64 containers by passing the
`--rosetta` flag. This allows native containers to benefit from being
able to execute the occasional amd64 executable in niche situations,
e.g., containers used for cross-compilation.
Resolves#391.
I considered adding a warning for passing `--rosetta` when it is not
necessary, but there wasn't obvious infrastructure for conveying the