Add xmlsec1 package (#226)

Closes #225

app/Dockerfile

@@ -14,6 +14,7 @@ RUN apt-get update \
       jq \
       netcat \
       ca-certificates \
+      xmlsec1 \
     && rm -rf /var/lib/apt/lists/* /var/cache/apt/archives/*.deb /var/cache/apt/archives/partial/*.deb /var/cache/apt/*.bin
 
 # Get Mattermost