Dockerfile: add package texinfo

In preparation of adding the infodocs target to our CI install the
prerequisite texinfo software package.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/tools/docker/Dockerfile b/tools/docker/Dockerfile
index 2408788..c92eeb5 100644
--- a/tools/docker/Dockerfile
+++ b/tools/docker/Dockerfile
@@ -109,6 +109,7 @@
 	srecord \
 	sudo \
 	swig \
+	texinfo \
 	util-linux \
 	uuid-dev \
 	virtualenv \