[Suggestion] Prominently warn or error if on arm with no platform specified about dockerfile-rails HOT 3 CLOSED

This is a direct follow up to #27. We got --jemalloc working nicely after the fix.

However, deploying to Fly we were still getting errors in our logs:

ERROR: ld.so: object '/usr/lib/aarch64-linux-gnu/libjemalloc.so.2' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.

We realized we need to also specify the --platform linux/amd64 which then sets LD_PRELOAD to the expected path properly:
LD_PRELOAD="/usr/lib/x86_64-linux-gnu/libjemalloc.so.2"

from dockerfile-rails.

I'm actually leaning towards erroring.

You saw this in the logs and were able to work backwards and figure it out. Many won't notice.

It is easy for me to update the fly docs to suggest setting both at the same time, so for most users it will be a matter of copying/pasting a slightly longer string.

To not break users who are currently deployed, I think --platform should default if --jemalloc had previously been set

from dockerfile-rails.

This is no longer relevant given the following change: 85822c2

from dockerfile-rails.