Set php.ini based on environment variables

[andrewnicols]

Following from #162 it would be good to include an init script (perhaps in a different location so as not to conflict) which allows the setting of php.ini settings from environment variables. For example:

docker run \
  -e PHPINI-upload_max_filesize=200M \
  -e PHPINI-post_max_size=208M \
  ...

We can fetch all env vars matching a prefix in bash and create a stub php.ini to place them into.