* Remove php src the right way for lightweight containers.
* Adapt docker-php-ext-* scripts to check exts against /available-exts generated file.
* Improve docker-php-source script
- swap spaces to tabs to match
- use gnu tar for `--strip-components`
- cache known extension list in `/usr/src/`
- remove extra output
* Changes per tianon's comments
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
```
