gitlab-ci: make sure EXIF is stripped from all included images

EXIF state can be used as an exploit vector and for malicioius code
delivery.  Neither fdroidclient nor fdroid-website use the EXIF data, so
just strip it entirely.  'exiftool' is available in most distros.

.gitlab-ci.yml

@@ -31,4 +31,10 @@ lint:
           fdroid rewritemeta $CHANGED;
           git --no-pager diff --color=always;
       }
+    - apt-get -qy update
+    - apt-get -qy install --no-install-recommends exiftool
+    - exiftool -all= `find metadata/ -name '*.jp*g' -o -name '*.png'`
+    - echo "these images have EXIF that must be stripped:"
+    - git --no-pager diff --stat
+    - git --no-pager diff --name-only --exit-code || export EXITVALUE=1
     - exit $EXITVALUE