New extension: blackfire.io probe

[insekticid]

• https://blackfire.io/docs/integrations/docker
• https://blog.blackfire.io/alpine-linux-support.html

RUN version=$(php -r "echo PHP_MAJOR_VERSION.PHP_MINOR_VERSION;") \
     && curl -A "Docker" -o /tmp/blackfire-probe.tar.gz -D - -L -s https://blackfire.io/api/v1/releases/probe/php/alpine/amd64/$version \
     && tar zxpf /tmp/blackfire-probe.tar.gz -C /tmp \
     && mv /tmp/blackfire-*.so $(php -r "echo ini_get('extension_dir');")/blackfire.so \
     && printf "extension=blackfire.so\nblackfire.agent_socket=tcp://blackfire:8707\n" > $PHP_INI_DIR/conf.d/blackfire.ini