Add arm64 alternative to frapsoft/openssl

[DRK3]

The generate-test-keys Makefile target uses frapsoft/openssl, which only has an amd64 version. While this does seem to work currently (as it seems that Docker makes use of QEMU to support emulation of non-native architectures), it would still be a good idea to find an alternative that supports both amd64 and arm64 natively for better performance and to avoid potential issues.