Use debian instead of alpine

This is related to issues with iconv on alpine linux (see [1]). I
noticed that the content file `tipuesearch_content.json` didn't contain
the sanitized text, but only the boolean `false`. I tracked it down to
the `iconv` call. After migration to the debian docker image it is
working fine. Build was started using `docker run --rm -it -w /build -v
"$PWD":/build daux/daux.io daux generate`.

[1] https://github.com/docker-library/php/issues/428

Dockerfile

@@ -1,8 +1,8 @@
 FROM composer:1.7.2 AS composer
 
-FROM php:7-alpine
+FROM php:7-stretch
 
-RUN apk info && apk add --no-cache unzip icu-dev
+RUN apt-get update && apt-get install -y libicu-dev git unzip
 RUN docker-php-ext-configure intl \
 && docker-php-ext-install intl