Add redis library to docker image; mention xapian is installed

Support using redis for session data.

xapian has to match the version of xapian libraries installed in the
image. So it is nt installed by requirements.txt. Make note in
requirements.txt that it is installed.

Author: rouilj

scripts/Docker/requirements.txt

@@ -2,9 +2,11 @@
 pytz
 # indexer
 Whoosh
+# xapian is also installed
 # extra database support
 psycopg2
 mysqlclient
+redis
 # encryption
 gpg
 # java web tokens