- The linux base64 command is different, so avoid using it at all.
On linux the default is to split the output into multiple lines,
which messes up the integration-test-env file. The flag used to
disable this behavior on linux ("-w0") does not exist on MacOS's
base64.
- On debian linux, the latest version of Docker from apt-get still
requires DOCKER_BUILDKIT=1 or else it barfs.