Fix issue with iconv in PHP image and use Alpine binary of ci-helper

[mremi]

See docker-library/php#240

Before:

php -d error_reporting=22527 -d display_errors=1 -r 'var_dump(iconv("UTF-8", "UTF-8//IGNORE", "This is the Euro symbol '\''€'\''."));'
Notice: iconv(): Wrong charset, conversion from `UTF-8' to `UTF-8//IGNORE' is not allowed in Command line code on line 1

After:

php -d error_reporting=22527 -d display_errors=1 -r 'var_dump(iconv("UTF-8", "UTF-8//IGNORE", "This is the Euro symbol '\''€'\''."));'
Command line code:1:
string(30) "This is the Euro symbol '€'."