Add a tool to generate the extensions.json file (#83)

Instead of manually maintaining the file, there is now a tool that grab
all repositories defined in the repositories.json file and browse through
all repositories to find extensions.
To do that, I've added a container that runs only php-cli to trigger said
tool.

The next step will be to trigger that tool automatically on a regular basis
and to update the current repository when there are changes.

Docker/Dockerfile

@@ -0,0 +1,3 @@
+FROM php:8.1-cli-alpine
+
+RUN apk add --no-cache git