docker: add pandoc package
docker: add pandoc package

Needed for documentation generation (which uses the same container
image as continuous integration).
Message-Id: <fbd88b97001c5db0a655aa0a72b98f7395660c4a@scylladb.com>

avikivity/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,scylladb/seastar,syuu1228/seastar