Dockerfile: Add doxygen package

Since #1184 `./configure` checks for the presence of doxygen, but the
Docker image does not include it, hence the build failed. (closes #1217)

Signed-off-by: Julien Hachenberger <julien.hachenberger@sit.fraunhofer.de>

diff --git a/Dockerfile b/Dockerfile
index b3d44e9..1ad7c80 100644
--- a/Dockerfile
+++ b/Dockerfile

@@ -4,6 +4,7 @@
     autoconf-archive \
     automake \
     build-essential \
+    doxygen \
     g++ \
     gcc \
     git \