Ubuntu 20.04 ("focal") on armhf (arm32) currently has an issue where it looks
like seccomp is blocking a syscall that's used when installing libc6:

    docker run -e DEBIAN_FRONTEND=noninteractive --rm arm32v7/ubuntu:focal sh -c 'apt-get -q update && apt-get install -y libc6'

    ...
    Preparing to unpack .../libc6_2.31-0ubuntu6_armhf.deb ...
    Checking for services that may need to be restarted...
    Checking init scripts...
    Checking for services that may need to be restarted...
    Checking init scripts...
    Nothing to restart.
    Unpacking libc6:armhf (2.31-0ubuntu6) over (2.30-0ubuntu3) ...
    tar: ./control: Cannot utime: Operation not permitted
    tar: ./md5sums: Cannot utime: Operation not permitted
    tar: ./shlibs: Cannot utime: Operation not permitted
    tar: ./symbols: Cannot utime: Operation not permitted
    tar: ./triggers: Cannot utime: Operation not permitted
    tar: .: Cannot utime: Operation not permitted
    tar: Exiting with failure status due to previous errors
    dpkg-deb: error: tar subprocess returned error exit status 2
    dpkg: error processing archive /var/cache/apt/archives/libcrypt1_1%3a4.4.10-10ubuntu4_armhf.deb (--unpack):
     dpkg-deb --control subprocess returned error exit status 2
    Errors were encountered while processing:
     /var/cache/apt/archives/libcrypt1_1%3a4.4.10-10ubuntu4_armhf.deb
    E: Sub-process /usr/bin/dpkg returned an error code (1)

The issue was reproduced on:

Image `arm32v7/ubuntu:focal@sha256:18100e418054ebe1be0fff4e514183f28088a0db409df081c3233dd22dcf4a15`

    Server: Docker Engine - Community
     Engine:
      Version:          19.03.5
      API version:      1.40 (minimum version 1.12)
      Go version:       go1.12.12
      Git commit:       633a0ea
      Built:            Wed Nov 13 07:32:48 2019
      OS/Arch:          linux/arm
      Experimental:     true
     containerd:
      Version:          1.2.10
      GitCommit:        b34a5c8af56e510852c35414db4c1f4fa6172339
     runc:
      Version:          1.0.0-rc8+dev
      GitCommit:        3e425f80a8c931f88e6d94a8c831b9d5aa481657
     docker-init:
      Version:          0.18.0
      GitCommit:        fec3683

    Info:

     Kernel Version: 4.4.127-mainline-rev1
     Operating System: Ubuntu 16.04.5 LTS
     OSType: linux
     Architecture: armv7l
     CPUs: 4
     Total Memory: 1.974GiB

This issue was also reported in:

    - docker/containerd-packaging pr 151
    - dotnet/dotnet-docker issue 1747
      > It's not clear whether https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1867431 is related.
      > So I've logged https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1867675 for this issue.

This patch disables building on Ubuntu 20.04

Signed-off-by: Sebastiaan van Stijn <[email protected]>