Stabilize bullseye image builds by simplifying apt repository bootstrap

[Copilot]

The failing GitHub Actions job was in the bullseye image build path, where the repository bootstrap step pulled transient Debian security packages that were returning 404s before OpenResty installation began. This updates the bullseye variants to use a smaller, explicit apt setup path and removes the fragile bootstrap dependencies.

• Bullseye apt bootstrap

  • Replace apt-key and add-apt-repository with an explicit keyring file in /etc/apt/trusted.gpg.d
  • Write the OpenResty apt source directly to /etc/apt/sources.list.d/openresty.list
  • Pin the distro explicitly to bullseye

• Dependency reduction

  • Remove bootstrap-only packages that were causing the failure path:
    • gnupg2
    • lsb-release
    • software-properties-common
  • Keep only the minimal packages needed to fetch and register the repository key

• Variants updated

  • bullseye/Dockerfile
  • bullseye/Dockerfile.debug
  • bullseye/Dockerfile.valgrind

RUN ... \
    && wget -qO /etc/apt/trusted.gpg.d/openresty-keyring.gpg ${RESTY_APT_PGP} \
    && echo "deb [signed-by=/etc/apt/trusted.gpg.d/openresty-keyring.gpg] ${RESTY_APT_REPO} bullseye openresty" \
       | tee /etc/apt/sources.list.d/openresty.list \
    && apt-get update \
    && apt-get install -y --no-install-recommends openresty${RESTY_DEB_FLAVOR}${RESTY_DEB_VERSION}