Add exiftool to documentation and the Dockerfile

This binary is required by the new file metadata app.

Signed-off-by: Roberto Rosario <Roberto.Rosario@mayan-edms.com>

docker/Dockerfile

@@ -21,6 +21,7 @@ RUN if [ "${APT_PROXY}" ]; then echo "Acquire::http { Proxy \"http://${APT_PROXY
 # Install base Ubuntu libraries
 RUN apt-get update && \
 apt-get install -y --no-install-recommends \
+        exiftool \
         g++ \
         gcc \
         ghostscript \