Issue #266 - Upgrade Firefox to version 75.0

* Removed the Alpine Edge repository to avoid package conflicts

Author: ppodgorsek

Dockerfile

@@ -45,10 +45,7 @@ COPY bin/chromium-browser.sh /opt/robotframework/bin/chromium-browser
 COPY bin/run-tests-in-virtual-screen.sh /opt/robotframework/bin/
 
 # Install system dependencies
-RUN echo "http://dl-cdn.alpinelinux.org/alpine/edge/main" >> /etc/apk/repositories \
-  && echo "http://dl-cdn.alpinelinux.org/alpine/edge/community" >> /etc/apk/repositories \
-  && echo "http://dl-cdn.alpinelinux.org/alpine/edge/testing" >> /etc/apk/repositories \
-  && apk update \
+RUN apk update \
   && apk --no-cache upgrade \
   && apk --no-cache --virtual .build-deps add \
     gcc \