recommend DOCKER_DEFAULT_PLATFORM for macos users

[mr-c]

If MacOS cwltool users want to run tools/workflows that reference software containers that only support amd64 (64-bit x86), but they have an Apple Silicon (aarch64/arm64) CPU, then they will benefit from exporting the following environment variable.

export DOCKER_DEFAULT_PLATFORM=linux/amd64

Sources:

• https://forums.docker.com/t/docker-default-platform-should-be-more-obvious-to-apple-silicon-users-everyone/120805
• https://docs.docker.com/reference/cli/docker/#environment-variables

[mr-c]

fixed in #2064