Docker - hotfix building docker image (php version 7.2, don't use 7.3)

https://travis-ci.org/EdgedesignCZ/phpqa/jobs/527347724#L1916
Your requirements could not be resolved to an installable set of packages.
  Problem 1
    - Installation request for friendsofphp/php-cs-fixer v2.2.20 -> satisfiable by friendsofphp/php-cs-fixer[v2.2.20].
    - friendsofphp/php-cs-fixer v2.2.20 requires php ^5.3.6 || >=7.0 <7.3 -> your PHP version (7.3.4) does not satisfy that requirement.

.docker/Dockerfile

@@ -1,4 +1,4 @@
-FROM composer
+FROM composer:1.8.0
 
 RUN apk add --update libxslt-dev && \
     docker-php-ext-install xsl