docs to hold maintainer build platforms

Signed-off-by: Avi Deitcher <avi@deitcher.net>

docs/developer-setup.md

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+# Build Platforms
+
+This document describes how to install and maintain a LinuxKit development platform. It will grow over time.
+
+The LinuxKit team also maintains several Linux-based build platforms. These are donated by Equinix Metal (arm64) and IBM (s390x).
+
+## Platform-Specific Installation
+
+### arm64 and amd64
+
+The `amd64` and `arm64` platforms are fully supported by most OS vendors and Docker. Just upgrade to the latest OS and install the latest Docker using the
+packaging tools. As of this writing, that is:
+
+* Ubuntu/Debian with `apt`
+* RHEL/CentOS/Fedora with `yum`. For any of these, use the CentOS 7/8 packages as released by Docker.
+
+Docker does not recommend that you using the packages released by the OS vendors, as those tend to be out of date. Follow the instructions
+[from Docker](https://docs.docker.com/engine/install/).
+
+### s390x
+
+The s390x has modern versions of most OSes, including RHEL and Ubuntu, but does not have recent versions of docker, neither as
+`apt` packages for Ubuntu, nor as static downloads. In any case, these static downloads mostly are replicas.
+
+This section describes how to install modern versions of Docker on these platforms.
+
+#### RHEL
+
+RHEL 7 on s390x only has releases from Docker. Follow the instructions from Docker to install. The rpm packages for RHEL are available at
+https://download.docker.com/linux/rhel/
+
+#### Ubuntu
+
+Docker does not release packages for Ubuntu on s390x. The most recent release was for Ubuntu 18.04 Bionic, with Docker version 18.06.3.
+This is quite old, and does not support modern capabilities, e.g. buildkit.
+
+To install a more modern version:
+
+1. Upgrade any dependent apt packages `apt upgrade`
+1. Upgrade the operating system to your desired version `do-release-upgrade -d`. Note that you can set which versions to suggest via changing `/etc/update-manager/release-upgrades`
+1. Download the necessary rpms (yes, rpms) from the Docker RHEL7 site. These are available [here](https://download.docker.com/linux/rhel/7/s390x/stable/Packages/). You need the following packages:
+   * `containerd.io-*.rpm`
+   * `docker-ce-*.rpm`
+   * `docker-ce-cli-*.rpm`
+1. Install alien: `apt install alien`
+1. Convert each package to a dpkg `alien --scripts <source-rpm-file.rpm>`
+1. Install each package with `dpkg -i <source-dpkg>.dpkg`. Dependency management is not great, so we recommend installing them in order:
+   1. `containerd.io`
+   1. `docker-ce`
+   1. `docker-ce-cli`
+1. Install devmapper `apt install libdevmapper-dev`
+1. Check the missing version of libdevmapper, if any, with `ldd /usr/bin/dockerd`. In our example, it needs `libdevmapper.so.1.02`
+1. Ensure that the library can be found where needed via `cd /lib/s390x-linux-gnu/ && ln -s $(ls -1 libdevmapper.so.*) libdevmapper.so.1.02`
+1. Check again that dockerd is ok: `ldd /usr/bin/dockerd`
+1. Start docker `system ctl restart docker`
+1. Check that everything works:
+   * `docker version`
+   * `docker run --rm hello-world`
+
+## Common Notes
+
+On all platforms, if you want to run tests, you will need:
+
+* `jq`
+* `expect`
+* `qemu-kvm`
+
+These should be installed using your normal platform package installation, e.g. `apt install -y jq expect qemu-kvm`.
+
+You also will need `rtf`, which can be installed with `make bin/rtf && make install`.
+
+For pushing our kernels, you will need [manifest-tool](http://github.com/estesp/manifest-tool), which can be installed with
+`make bin/manifest-tool && make install`.
+
+Finally, to enable your regular user to run the tools, we recommend:
+
+```
+usermod -aG docker $USER
+usermod -aG kvm $USER
+usermod -aG sudo $USER
+```