Fix Linux instructions

The following discussion from !90 (merged) should be addressed:

• @helmich started a discussion: (+1 comment)

  These instructions do not work under Linux. When using the bash shell I think it should be this:

  echo "condausername:condapassword" > conda_download_key.txt
  export DOCKER_BUILDKIT=1
  docker build --secret id=conda_download_key,src="./conda_download_key.txt" -t micadoconda .