Installing Blackfire

[moufmouf]

As proposed by @mnapoli in #60 , this PR installs the Blackfire extension.

That would be awesome if Blackfire was preinstalled in the Docker images. That would allow to profile PHP scripts very easily.

To make that happen the following needs to be done in the Docker images:

• install the PHP extension: https://blackfire.io/docs/integrations/docker#enabling-the-php-probe
• install the Blackfire CLI tool in the image too (to profile PHP scripts): https://blackfire.io/docs/integrations/docker#using-the-client-for-cli-profiling

Users not using Blackfire should not be impacted because the extension will not do anything. (it could also be possible to make the extension opt-in through an environment variable, that would be even cleaner)

Users using Blackfire will need to:

• run the agent, e.g. through Docker-Compose: https://blackfire.io/docs/integrations/docker#running-the-agent
• define environment variables with their credentials (https://blackfire.io/docs/integrations/docker)
• install the browser extension (https://blackfire.io/docs/up-and-running/installation)

The Blackfire extension can be enabled using the PHP_EXTENSION_XDEBUG environment variable.

If Xebug and Blackfire are enabled at the same time, a Warning will be issued on container startup.

The blackfire agent URL is customizable via the BLACKFIRE_AGENT environment variable.

The Blackfire CLI is also installed out of the box in the image.