Cockpit doors keep threats out, but what if the threat is within?

A flydubai pilot has been accused of attacking his colleague in the cockpit in an alleged attempt to seize control of the plane and crash it. With reports the alleged attacker had been grounded at another airline over security concerns, should there be more information sharing between airlines? The cockpit doors, strengthened after September 11, also complicate matters, as they can keep rescuers out as well as potential hijackers. So what can airlines and regulators do about the risk of a radicalised or mentally unwell pilot?