By default, apt-get autoremove won't remove packages that are recommended or
suggested by *any* installed packages, not just the packages that
auto-installed them.
For example, the following makes WordPress work:
```Dockerfile
RUN apt-get update && apt-get install -y libpng12-dev && rm -rf /var/lib/apt/lists/* \
&& docker-php-ext-install gd \
&& apt-get purge --auto-remove -y libpng12-dev
RUN docker-php-ext-install mysqli
```
This results in a dramatic reduction of image size:
```console
$ docker images php
REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE
php 5.4 6b03f428509e 3 hours ago 354.3 MB
php 5.4-apache 2bd126f2c469 2 minutes ago 394.8 MB
php 5.5 19a5113df69a 9 minutes ago 356.4 MB
php 5.5-apache 6c892721307f 2 minutes ago 396.9 MB
php 5.6 69aead156fe7 10 minutes ago 356.8 MB
php 5.6-apache 2d0bae628aaf About an hour ago 397.3 MB
```
vs the old:
```console
$ docker images php
REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE
php 5.4 e760b9bd2c6c 2 days ago 736.9 MB
php 5.4-apache 62c23e8f1704 2 days ago 811.8 MB
php 5.5 156d2a323bc0 2 days ago 739.8 MB
php 5.5-apache 3014d0f81b29 2 days ago 814.7 MB
php 5.6 dab86418397c 2 days ago 740.2 MB
php 5.6-apache b3c4a0633954 2 days ago 815.1 MB
```
