For anyone concerned that authorities force you to give up your password to them (thus allowing them to image your device), you can pair your iPhone to a computer with a MDM (managed device profile), which will prevent any other device from connecting to it. iOS security researcher (now Apple employee) Jonathan Zdziarski has 2 blog posts on this:
Counter-Forensics: Pair-Lock Your Device with Apple’s Configurator:
https://www.zdziarski.com/blog/?p=2589
Protecting Your Data at a Border Crossing: