Comment 3 for bug 1872854

Sorry for sloppy bug report. Here are exact steps to reproduce on a Ubuntu 18.04 host (arch amd64) running s390x in a Docker container. The bug is not repeatable on amd64 since the s390x packages available on Bionic is slightly different.

# Install s390x emulators
sudo apt install qemu-user-static qemu-system-s390x

# Enable experimental mode in Docker
cat /etc/docker/daemon.json
{
"experimental": true
}
sudo systemctl restart docker

# Pull s390x Bionic
docker pull --platform linux/s390x ubuntu:bionic

# Prepare and build container
mkdir s390x; cd s390x
cp /usr/bin/qemu-s390x-static .
cat Dockerfile
FROM ubuntu:bionic
COPY ./qemu-s390x-static /usr/bin/qemu-s390x-static
RUN uname -a
RUN apt-get update -y && \
    apt-get install --yes --no-install-suggests --no-install-recommends software-properties-common curl gpg gpg-agent dirmngr ca-certificates && \
    echo 'deb http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu bionic main' >> /etc/apt/sources.list.d/toolchain.list && \
    apt-key adv --keyserver keyserver.ubuntu.com --recv 60C317803A41BA51845E371A1E9377A2BA9EF27F && \
    apt-get update -y
RUN apt-get --no-install-suggests --no-install-recommends install clang-7 clang-8 g++-5 g++-7 g++-8 g++-9

# Run
docker build --tag ubuntu:bionic-s390x .

# Result
Step 3/3 : RUN apt-get --no-install-suggests --no-install-recommends install clang-7 clang-8 g++-5 g++-7 g++-8 g++-9
 ---> Running in c3bd447b3aad
Reading package lists...
Building dependency tree...
Reading state information...
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 clang-7 : Depends: libstdc++-7-dev but it is not going to be installed
           Depends: libgcc-7-dev but it is not going to be installed
           Depends: libobjc-7-dev but it is not going to be installed
 clang-8 : Depends: libstdc++-7-dev but it is not going to be installed
           Depends: libgcc-7-dev but it is not going to be installed
           Depends: libobjc-7-dev but it is not going to be installed
 g++-7 : Depends: gcc-7 (= 7.5.0-3ubuntu1~18.04) but it is not going to be installed
         Depends: libstdc++-7-dev (= 7.5.0-3ubuntu1~18.04) but it is not going to be installed
 g++-9 : Depends: gcc-9 (= 9.2.1-17ubuntu1~18.04.1) but it is not going to be installed
         Depends: libstdc++-9-dev (= 9.2.1-17ubuntu1~18.04.1) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.