Use fewer layers vor environment variables

[solidnerd]

No description provided.

[jnbt]

Thanks for the contribution, but it fails when building due to the single line ENV statement:

Step 13 : RUN curl -O -L $ROUNDCUBE_DOWNLOAD  && tar xzf $ROUNDCUBE_VARIANT.tar.gz  && rm -r $ROUNDCUBE_DIR  && mv $ROUNDCUBE_PACKAGE $ROUNDCUBE_DIR  && rm -rf $ROUNDCUBE_PACKAGE  && echo '<?php\n$config = array();\n' > $ROUNDCUBE_DIR/config/config.inc.php  && rm -fr $ROUNDCUBE_DIR/installer  && mkdir -p $ROUNDCUBE_DIR/config/custom  && chown -R www-data:www-data $ROUNDCUBE_DIR
 ---> Running in fd0bb94f4040
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100    21    0    21    0     0     34      0 --:--:-- --:--:-- --:--:--    34
tar (child): -complete.tar.gz: Cannot open: No such file or directory
tar (child): Error is not recoverable: exiting now
tar: Child returned status 2
tar: Error is not recoverable: exiting now