docker STR to observe LE failure on FPF apt-test repo

Dockerfile

# Intentionally using an old tag, to test old Focal packages
# See here for options: https://quay.io/repository/freedomofpress/sd-docker-builder-focal?tab=tags
# See related expiry info for LE HTTPS certs here:
# https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/
FROM quay.io/freedomofpress/sd-docker-builder-focal:2021_04_14

RUN apt-get update && apt-get install -y curl apt-transport-https gnupg2

# Uncomment the upgrade task below to see a working build!
# RUN apt-get upgrade -y

RUN echo "deb [arch=amd64] https://apt-test.freedom.press focal main" | tee /etc/apt/sources.list.d/fpf-apt-test-repo.list

RUN curl -s https://raw.githubusercontent.com/freedomofpress/securedrop/develop/install_files/ansible-base/roles/install-fpf-repo/files/apt-test-signing-key.pub | apt-key add -

# The following line fails, with:
#
#   9 0.711 Err:4 https://apt-test.freedom.press focal Release
#   9 0.711   Certificate verification failed: The certificate is NOT trusted.
#   The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 206.81.1.102 443]
#
RUN apt-get update