Update the trusted CA certificates

Name ca-certificates in the base stage's apt install so the image takes the
distribution's current package rather than the one in the base image, and fail
the build if either withdrawn root (emSign ECC Root CA - G3, AAA Certificate
Services) is still in the store Ruby's OpenSSL loads.

Dockerfile

@@ -9,10 +9,23 @@ WORKDIR /rails
 
 # Install base packages
 RUN apt-get update -qq && \
-    apt-get install --no-install-recommends -y curl libsqlite3-0 libvips libjemalloc2 ffmpeg redis && \
+    apt-get install --no-install-recommends -y ca-certificates curl libsqlite3-0 libvips libjemalloc2 ffmpeg redis && \
     ln -s /usr/lib/$(uname -m)-linux-gnu/libjemalloc.so.2 /usr/local/lib/libjemalloc.so && \
     rm -rf /var/lib/apt/lists /var/cache/apt/archive
 
+# Naming ca-certificates above takes the distribution's current trusted roots rather than the
+# ones frozen into the base image. Fail the build if either of these withdrawn roots is still
+# where Ruby's OpenSSL looks for trusted certificates.
+RUN ruby -ropenssl -e ' \
+    file = ENV[OpenSSL::X509::DEFAULT_CERT_FILE_ENV] || OpenSSL::X509::DEFAULT_CERT_FILE; \
+    dir = ENV[OpenSSL::X509::DEFAULT_CERT_DIR_ENV] || OpenSSL::X509::DEFAULT_CERT_DIR; \
+    pems = [file, *Dir.glob(File.join(dir, "*"))].select { |path| File.file?(path) } \
+      .flat_map { |path| File.read(path).scan(/-----BEGIN CERTIFICATE-----.+?-----END CERTIFICATE-----/m) }; \
+    subjects = pems.map { |pem| OpenSSL::X509::Certificate.new(pem).subject.to_s }.uniq; \
+    abort "Only #{subjects.size} roots in #{file} and #{dir}" if subjects.size < 100; \
+    withdrawn = subjects.grep(%r{/CN=(emSign ECC Root CA - G3|AAA Certificate Services)\z}); \
+    abort "Withdrawn roots still trusted: #{withdrawn}" if withdrawn.any?'
+
 # Set production environment
 ENV RAILS_ENV="production" \
     BUNDLE_DEPLOYMENT="1" \