Wrong check for plugins ZIPs

[camuffo]

The Dockerfile checks for zip files with [ -f *zip ].
If * is expanded to more that one file, it becomes invalid syntax.

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#167 fixes this by using ls.

This screenshots compare contents of two example images.
Both built with two plugins ZIPs in place:

ls resources/geoserver-plugins/
geoserver-2.26.2-mbstyle-plugin.zip  geoserver-2.26-SNAPSHOT-mbtiles-store-plugin.zip

Built with
docker build --build-arg GEOSERVER_WEBAPP_SRC=./geoserver.war --build-arg PLUG_IN_PATHS=./resources/geoserver-plugins --no-cache .

Left: built on branch master / Right: built on branch fix-many-plugins-zips