Remove build dependencies after docker-php-ext-install

[J0WI]

Since removing build dependencies after installing PHP extensions (e.g. docker-library/wordpress#267) seems to be a common task it would make sense to have such a functionality in the docker-php-ext-* scripts.

[ebuildy]

/usr/src/php.tar.xz can be removed too ?

docker run --rm php:7.2.5-cli-alpine3.7 ls -alh /usr/src => 11.5M (16% of the total image size)

[J0WI]

Nope, that is used here.

[ebuildy]

yes, I mean, once we have installed the dependencies, we could remove it too (not use at run-time).

[yosifkit]

As for deleting the source, that would be a duplicate of #488.

For auto-removing build dependencies that we add, that is done in the Alpine images:

php/docker-php-ext-install

Lines 116 to 118 in 4017680

	if [ "$pm" = 'apk' ] && [ -n "$apkDel" ]; then
	apk del $apkDel
	fi

We don't auto-add build dependencies in Debian since any needed for phpize are already installed. See #513 (comment) and #438 for why we don't auto add/remove packages in Debian.

As for any other dependencies that could be installed in a Dockerfile, since we didn't add them, we can't reasonably detect that they can be removed.