You need to add the universe repository:

RUN apt-get update &&\
 apt-get install -y software-properties-common &&\
 add-apt-repository universe &&\
 apt-get install -y php-apcu

If you get a "Error: 'universe' invalid" then you need to use the full source line:

 sudo add-apt-repository "deb http://archive.ubuntu.com/ubuntu $(lsb_release -sc) universe"

instead of "universe" in your apt-add-repository line.