Fix docker build for EOL Debian release

signer_client.Dockerfile

@@ -3,7 +3,11 @@ FROM debian/eol:jessie
 COPY testca/root/ca.crt.pem /usr/local/share/ca-certificates/testca_root.crt
 COPY testca/class3/ca.crt.pem /usr/local/share/ca-certificates/testca_class3.crt
 
-RUN apt-get update \
+RUN sed -i \
+      -e 's@\(^deb http://deb\..*\)$@#\1@' \
+      -e 's@\(^deb http://security\..*\)$@deb http://archive.debian.org/debian-security jessie/updates main@' \
+      /etc/apt/sources.list \
+    && apt-get update \
     && DEBIAN_FRONTEND=noninteractive \
     apt-get install -y --no-install-recommends \
     ca-certificates \