fix: Debian Stretch repositories signing keys were also archived

Dockerfile

@@ -3,10 +3,12 @@ FROM ${BASE_IMAGE}
 LABEL maintainer="dev@chialab.io"
 
 # Fix Debian 9 (Stretch) source list, because it has been moved to archive
+# Paranoid people should manually download and verify the signing key from https://ftp-master.debian.org/keys.html
 RUN if [ "$(grep '^VERSION_ID=' /etc/os-release | cut -d '=' -f 2 | tr -d '"')" -eq "9" ]; then \
         sed -i -e 's/deb.debian.org/archive.debian.org/g' \
                -e 's/security.debian.org/archive.debian.org/g' \
                -e '/stretch-updates/d' /etc/apt/sources.list; \
+        apt-get update && apt-get install -y --allow-unauthenticated debian-archive-keyring; \
     fi
 
 # Download script to install PHP extensions and dependencies