I have a legal question concerning the use of the OpenJDK binary in Docker containers specifically for commercial purposes in SAAS context. Is it legal? Do I need a "special" license from Oracle to use it in this way?
You need to follow the licensing for each software component or package you're using in your final product.
For reference, these are the components and licensing for all the 3rd party software in our final products: https://www.docker.com/components-licenses.
OpenJDK is under the 'GNU Public V2' license. See details at https://en.wikipedia.org/wiki/OpenJDK.