Add php7-mbstring extension to fix composer installation

[svemir]

Without this extension, running build.rb on OS-X fails with:

Step 2 : RUN curl -sS https://getcomposer.org/installer | php ---> Running in b918100ad154 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 iconv OR mbstring extension is required and both are missing. Install either of them or recompile php without --disable-iconv

[svemir]

Perhaps this type of pull request makes no sense for the actual usage of iron/php? I was looking at it as a general minimal php setup.

[treeder]

Is that required only on the build image or typically used in production?

[svemir]

It seems to be required by composer. I guess you could install it temporarily, install composer temporarily, use composer to install all your php code and create autoload files, and then remove both the extension and composer itself?

Obviously that would not work for such a generic image. I am not sure how exactly this php image is used within iron worker. For our own purposes we are building something much more customized anyway.