Fix example for custom docker images with plugins instlled

fixes roundcube#159

README.md

@@ -126,32 +126,24 @@ docker build -t roundcubemail .
 You can also create your own Docker image by extending from this image.
 
 For instance, you could extend this image to add composer and install requirements for builtin plugins or even external plugins:
+
 ```Dockerfile
 FROM roundcube/roundcubemail:latest
 
-COPY --from=composer:2 /usr/bin/composer /usr/bin/composer
-
 RUN set -ex; \
     apt-get update; \
     apt-get install -y --no-install-recommends \
         git \
     ; \
     \
-    mv /usr/src/roundcubemail/composer.json-dist /usr/src/roundcubemail/composer.json; \
-    \
     composer \
         --working-dir=/usr/src/roundcubemail/ \
-        --prefer-dist --prefer-stable \
-        --no-update --no-interaction \
-        --optimize-autoloader --apcu-autoloader \
+        --prefer-dist \
+        --prefer-stable \
+        --update-no-dev \
+        --no-interaction \
+        --optimize-autoloader \
         require \
             johndoh/contextmenu \
     ; \
-    composer \
-        --working-dir=/usr/src/roundcubemail/ \
-        --prefer-dist --no-dev \
-        --no-interaction \
-        --optimize-autoloader --apcu-autoloader \
-        update;
-
 ```

examples/README.md

@@ -31,7 +31,7 @@ which is used to install plugins. You can add and activate plugins by executing
 inside a running Roundcube container:
 
 ```
-$ docker exec -it roundcubemail composer.phar require johndoh/contextmenu --update-no-dev
+$ docker exec -it roundcubemail composer require johndoh/contextmenu --update-no-dev
 ```
 
 If you have mounted the container's volume `/var/www/html` the plugins installed persist on your host system. Otherwise they need to be (re-)installed every time you update or restart the Roundcube container.