add note on docker platform issue (#2064)

Co-authored-by: Michael R. Crusoe <[email protected]>

Author: Brilator

README.rst

@@ -189,6 +189,22 @@ and ``--tmp-outdir-prefix`` to somewhere under ``/Users``::
 
     $ cwl-runner --tmp-outdir-prefix=/Users/username/project --tmpdir-prefix=/Users/username/project wc-tool.cwl wc-job.json
 
+
+Docker default platform on macOS with Apple Silicon
+===================================================
+
+If macOS users want to run CWL tools/workflows using ``cwltool`` with Docker and their software containers only have support for amd64 (64-bit x86) CPUs, but they have an Apple Silicon (aarch64/arm64) CPU,
+they run into the error:
+
+  WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested.
+
+To fix this, export the following environment variable before executing `cwltool`:
+
+``export DOCKER_DEFAULT_PLATFORM=linux/amd64``
+
+To automatically have this variable set in the future, add the same command to ones respective shell profile (e.g. ``~/.zshrc``, ``~/.bash_profile``).
+
+
 Using uDocker
 =============