I wanted to try the --squash parameter for docker build on the Azure Pipelines platform.
I found it to be not enabled by default and a bit tricky to get working. (As of 25/03/2020).
It did work for me doing the following 2 things:
To enable experimental features in the Docker CLI (referred in docker version as Docker Client):
# Your job should have in the variables setting 'DOCKER_CLI_EXPERIMENTAL' set to 'enabled'