Add a signer variant with ancient base image

The production signer is using a really old base OS (Debian 4.0 Etch or
5.0 Lenny) with some packages like openssl from Debian 6.0 Stretch. This
commit uses the oldest available Docker base image to make a somewhat
reproducible variant of this OS.

signer.Dockerfile

@@ -1,8 +1,10 @@
-FROM debian:jessie
+# should be etch or lenny but dockerhub doesn't have such old versions
+FROM debian:squeeze
 
-RUN apt-get update \
+RUN echo "deb http://archive.debian.org/debian squeeze main" > /etc/apt/sources.list ; \
+    apt-get -o=Acquire::AllowInsecureRepositories=true update \
     && DEBIAN_FRONTEND=noninteractive \
-    apt-get install -y --no-install-recommends \
+    apt-get -o=APT::Get::AllowUnauthenticated=true install -y --no-install-recommends \
     gnupg \
     libdevice-serialport-perl \
     libdigest-sha-perl \