[CI] Try to make composer run

Dockerfile.linux.amd64

@@ -6,19 +6,20 @@ RUN set -e && ln -sf bash /bin/sh
 RUN set -e \
       && apt -y update \
       && apt -y upgrade \
-      && apt -y install --no-install-recommends --no-install-suggests ca-certificates composer git \
+      && apt -y install --no-install-recommends --no-install-suggests ca-certificates git \
       && apt -y autoremove \
       && apt clean \
       && rm -rf /var/lib/apt/lists/*
 
 # install plugins using composer
-RUN set -e && /usr/bin/composer \
-      --working-dir=/usr/src/roundcubemail/ \
-      --prefer-dist \
-      --prefer-stable \
-      --update-no-dev \
-      --no-interaction \
-      --optimize-autoloader \
-      --quiet \
-      require \
-        johndoh/contextmenu
+RUN set -e \
+      && /usr/bin/composer \
+        --working-dir=/usr/src/roundcubemail/ \
+        --prefer-dist \
+        --prefer-stable \
+        --update-no-dev \
+        --no-interaction \
+        --optimize-autoloader \
+        --quiet \
+        require \
+          johndoh/contextmenu