build(docker): add file package to clear error when building xapian

It looks like the error was non-fatal. The image I believe worked
fine. But, clean up the noise.

Author: rouilj

scripts/Docker/Dockerfile

@@ -51,6 +51,7 @@ RUN apk --no-cache upgrade
 # Can't use apk to add python packages as it installs 3.9 python version.
 #        g++ installs cc1plus needed by pip install
 RUN apk --no-cache add \
+    file \
     g++ \
     gcc \
     gpgme-dev \