Revert "added support for Docker's '--volumes ...:ro' readonly suffix"

This reverts commit 146126d83afd217b2c41ba4d0e0afb38ca5c0103.
Revert "added support for Docker's '--volumes ...:ro' readonly suffix"

This reverts commit c3bd5322bb37f7dc9d2d796cf22713e2e37a4390.