Use composer base image

Author: Greg Bowler

Dockerfile

@@ -1,9 +1,10 @@
-FROM phpstan/phpstan:0.12
+FROM composer:1.10
 
 LABEL version="1.0.0"
 LABEL repository="https://github.com/php-actions/phpstan"
 LABEL homepage="https://github.com/php-actions/phpstan"
 LABEL maintainer="Greg Bowler <greg.bowler@g105b.com>"
 
+RUN composer global require --no-progress phpstan/phpstan 0.12.*
 COPY entrypoint /usr/local/bin/entrypoint
 ENTRYPOINT ["/usr/local/bin/entrypoint"]