Return backwards compatibility to the way we handle /usr/src/php/ext/* (especially for user-supplied subdirectories looking to utilize docker-php-ext-* for non-core modules)

[tianon]

Closes #269
Closes #268
Closes #266

[tianon]

The gist of how this works is:

1. if /usr/src/php already exists when we start, we do not remove it (allows for dropping folders in /usr/src/php/ext/* as before)
2. if we create /usr/src/php ourselves via extract, we delete it
3. no more cache for existing extensions -- extracting is reasonably fast, and has to happen for actual build anyhow, so just do it sooner

[shouze]

LGTM

[yosifkit]

LGTM

[partounian]

Sorry but I'm a bit new to this, and these past few issues have me confused. If I need PECL to install modules would I just use docker-php-ext-* now?

[tianon]

No, pecl is absolutely still appropriate to use directly -- the only docker-php-ext-* script which should be used when installing from pecl is docker-php-ext-enable (if needed).

This PR simply adds back semi-support for the strange way folks were dumping extension sources into subdirectories of /usr/src/php/ext and using docker-php-ext-* to build/install them.

[partounian]

Oh okay, well previously I was getting an issue for phpize previously, and couldn't figure out how to instal the Alpine dev package for that to fix the issue. Any pointers?

[yosifkit]

@partounian, are you asking about figuring out which -dev is needed to install a specific php plugin (https://pkgs.alpinelinux.org/contents to find which package has the header file needed) or about installing general development tools for phpizing a plugin (RUN apk add --no-cache --virtual .phpize-deps $PHPIZE_DEPS && phpize ... && apk del .phpize-deps)?

[partounian]

Well I just need to install the plugins for APCu, and Graphicsmagick for PHP5.5. So my question is, what's the best way to get those installed? For gmagick I am already downloading the pkg with apk testing, but can't get the plugin installed with PECL.