Update php

Changes:

- docker-library/php@399ed12: Merge pull request docker-library/php#1264 from msierks-pcln/alpine-gnu-libiconv
- docker-library/php@b62fa33: Use gnu-libiconv for php iconv extension on alpine
- docker-library/php@7fcdbf3: Merge pull request docker-library/php#1262 from msierks-pcln/shared-iconv
- docker-library/php@85143a1: Update iconv extension to be shared, so it can be disabled if necessary

Author: docker-library-bot