Fix xmlsec build.

Author: jeffFranklin

Diff for: Dockerfile

@@ -1,7 +1,7 @@
 FROM python:3-slim
 
 RUN apt-get update && \
-    apt-get install -y libxmlsec1-dev build-essential libxmlsec1 libxmlsec1-openssl && \
+    apt-get install -y libxmlsec1-dev build-essential libxmlsec1 libxmlsec1-openssl pkg-config && \
     pip install -U xmlsec && \
     apt-get remove -y libxmlsec1-dev build-essential && \
     apt-get autoclean -y && \