php-apache docker insttall additional plugins, extension and enabling them, issue with -y --force-yes

 in docker file:

FROM php:7.1.2-apache

#

RUN docker-php-ext-install mysqli

#

RUN echo "deb http://archive.debian.org/debian stretch main" > /etc/apt/sources.list

#

RUN apt-key update

RUN apt-get update

RUN apt-get -y --force-yes install libpng-dev

RUN apt-get -y --force-yes install zlib1g-dev

RUN docker-php-ext-install mbstring

RUN docker-php-ext-install zip

RUN docker-php-ext-install gd

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php 7.1 needed for Invision community old version etc

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enabling extensions:

https://stackoverflow.com/questions/2283199/enabling-installing-gd-extension-without-gd

extension=gd.so 

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-y, --force-yes

https://askubuntu.com/questions/725442/warning-the-following-packages-cannot-be-authenticated

https://askubuntu.com/questions/523962/how-to-install-a-package-with-apt-without-the-do-you-want-to-continue-y-n-p

or apt-get, -y or --assume-yes work well (as @rinzwind explained) and I assume that's the best answer here. For many such interactive administrative operations, there is a similar command.
Another pretty generic way to do this is something like :
 $ echo "y" | sudo apt-get install edamame_biscuit