Ok, I understood what happens using the the acbuild run --debug option.
When
acbuild run -- echo 'root:screencast' | chpasswd
gets executed it returns Running: [echo root:screencast] , the pipe is executed on the host machine. To get the intended result it should be
acbuild run -- /bin/sh -c "echo 'root:screencast' | chpasswd"
or in generic form
acbuild run -- /bin/sh -c "<cmd with pipes>"
as explained here