Add the phar extension to PHP containers

[mikeschinkel]

Trying to use our containers on CircleCI I get

bash .circleci/install-composer.sh
Some settings on your machine make Composer unable to work properly.
Make sure that you fix the issues listed below and run this script again:

The phar extension is missing.
Install it or recompile php without --disable-phar

Here are a couple references:

• https://stackoverflow.com/a/22972639/102699
• https://stackoverflow.com/a/41967697/102699

Here is an article about PHAR:

• https://www.sitepoint.com/packaging-your-apps-with-phar/