Fatal error: Call to undefined function easter_date()

Seems like your php has been compiled without --enable-calendar support

http://ru.php.net/manual/en/calendar.installation.php

If you use a Dockerfile based on php:7.1, you can install the calendar extension easily:

RUN docker-php-ext-install calendar && docker-php-ext-configure calendar

I simply had to enable the calendar extension in php.ini:

extension = calendar.so