Undocumented environment variables

[butaca]

I found the following environment variables which are missing in https://fly.io/docs/reference/runtime-environment/

FLY_IMAGE_REF
FLY_MACHINE_ID
FLY_MACHINE_VERSION
FLY_PRIVATE_IP
FLY_PROCESS_GROUP
FLY_VM_MEMORY_MB

Since they aren't documented, I don't know exactly if they are safe to use.

[catflydotio]

Completed with #1059

[butaca]

Thank you! :)