Allow optimizing amd64 / x86 images on Mac M1 (arm64)

Expected Behavior

That's likely a feature request, but I feel that docker-slim would gain much broader audience if it would allow Apple Silicon users optimizing intel-based images. Since I don't have access to linux and optimizing via virtual machine would be a pain I would really love that feature!

Actual Behavior

Getting this mid-optimization (after successful http probing)

...
Removing intermediate container bf0ab38c5a2f
 ---> 770b6f665177
Step 137/141 : ENV BUILD_ID "12345"
 ---> Running in bfd44e811f58
Removing intermediate container bfd44e811f58
 ---> b7b860e814d1
Step 138/141 : ADD files.tar /

cmd=build log='optimized.image.build' event=LOG.END tag='amd64-image-to-optimize.slim'  ====================
cmd=build info=build.error status='optimized.image.build.error' error='failed to get destination image "sha256:...": image with reference sha256:... was found but does not match the specified platform: wanted linux/amd64, actual: linux/arm64
cmd=build state=exited code=33554435 version=darwin|Transformer|1.37.5-4-gd3b7a36|d3b7a368ba1441a59a07284a082b608eeb6d28aa|2022-04-11_12:32:00 ...

Specifications

• Version: docker-slim version darwin|Transformer|1.37.5-4-gd3b7a36|d3b7a368ba1441a59a07284a082b608eeb6d28aa|2022-04-11_12:32:00
• Platform: OS X 12.2 on Macbook Air with M1 chip

Start by reproducing docker-slim optimization on OS X with an M1 chip using a linux/amd64 image, following the reported failure after HTTP probing. Investigate the platform mismatch in the optimization and build flow; done means amd64 images can be optimized successfully on arm64 Macs without the reported image-reference error.