git clone git@github.com:docker-library/php.git docker-library-php
## Go to the specific version you're interested in
cd docker-library-php/7.1/fpm/alpine

## Edit the .Dockerfile.
## Change
## ENV PHP_EXTRA_CONFIGURE_ARGS --enable-fpm --with-fpm-user=www-data --with-fpm-group=www-data
## to
## ENV PHP_EXTRA_CONFIGURE_ARGS --enable-fpm --with-fpm-user=www-data --with-fpm-group=www-data --enable-debug
## Comment out or delete:
## && { find /usr/local/bin /usr/local/sbin -type f -perm +0111 -exec strip --strip-all '{}' + || true; } \
## && docker-php-source delete \

## Now build the container with a tag that make sense.
docker build -t sh41/php:php-7.1.6-fpm-alpine-debug .

## and optionally push to a repo.
## (might need to docker login)
docker push sh41/php:php-7.1.6-fpm-alpine-debug

## now use this newly built container in place of the non-debug one.