Introduce GRADLE_HOME env variable to the compose-web Dockerfile

ci/docker/compose-web/Dockerfile

@@ -23,8 +23,8 @@ RUN unzip -d /usr/local/bin ~/tmp/chromedriver_linux64.zip && rm ~/tmp/chromedri
 ARG GRADLE_VERSION=6.8.3
 RUN wget https://services.gradle.org/distributions/gradle-${GRADLE_VERSION}-bin.zip -P /tmp
 RUN unzip -d /opt/gradle /tmp/gradle-${GRADLE_VERSION}-bin.zip
-ARG GRADLE_HOME=/opt/gradle/gradle-${GRADLE_VERSION}
-ENV PATH=${GRADLE_HOME}/bin:$PATH
+ENV GRADLE_HOME=/opt/gradle/gradle-${GRADLE_VERSION}
+ENV PATH=$GRADLE_HOME/bin:$PATH
 
 ARG CHROME_VERSION="google-chrome-stable"
 RUN wget -q -O - https://dl-ssl.google.com/linux/linux_signing_key.pub | apt-key add -