Add instructions from <CAKNHny8ZXyzAD6fCa0WS2wDs_bxQcO0oKyFns3Q022yuCVkDKw@mail.gmail.com>.

README.md

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+# make-live
+
+## Building a PureOS ISO image
+
+    From: Matthias Klumpp <matthias.klumpp@puri.sm>
+    Date: Wed, 6 Mar 2019 22:04:37 +0100
+    Subject: Re: [PureOS] Building my own PureOS ISO image?
+    Message-ID: <CAKNHny8ZXyzAD6fCa0WS2wDs_bxQcO0oKyFns3Q022yuCVkDKw@mail.gmail.com>
+
+    > [..]
+
+    That should be pretty easy. The basic process is the following (see
+    below for replicating almost 100% of what our builders do):
+
+    1) Create a PureOS green chroot (just debootstrap green and chroot into PureOS)
+    1.1) /!\ All following steps happen inside of the PureOS chroot
+    2) Run `apt install live-build git ca-certificates`
+    3) cd /srv && git clone https://source.puri.sm/pureos/infra/make-live.git
+    4) cd /srv/make-live
+    5) Set the FLAVOR environment variable to what flavor of PureOS you
+    want to build. Possible values are:
+        * gnome-live
+        * gnome-oem
+        * plasma-live
+       E.g. `export FLAVOR="gnome-oem"`.
+       If no FLAVOR variable is set, the GNOME Live Flavor will be built by default
+    6) Run `lb config`
+    7) Run `lb build`
+    8) Have fun with a fresh new PureOS image!
+
+    These steps can be sped up a lot and be automated by using debspawn,
+    which is used to create the official PureOS images in a controlled
+    environment.
+    In order to use debspawn, just follow these steps:
+    1) Run `sudo apt install debspawn`
+    2) Then run `debspawn create green`
+    3) Create a new "build-pureos.sh" file with the following contents:
+    ```
+    #!/bin/sh
+    set -e
+    set -x
+
+    export FLAVOR="gnome-live"
+    export DEBIAN_FRONTEND=noninteractive
+    apt-get install -y git ca-certificates
+    apt-get install -y live-build
+
+    git clone --depth=2 https://source.puri.sm/pureos/infra/make-live.git
+    /srv/build/lb
+    cd /srv/build/lb/
+
+    lb config
+    lb build
+
+    b2sum *.iso *.contents *.zsync *.packages > checksums.b2sum
+    sha256sum *.iso *.contents *.zsync *.packages > checksums.sha256sum
+    mv *.iso /srv/artifacts
+    mv -f *.zsync /srv/artifacts
+    mv -f *.contents /srv/artifacts
+    mv -f *.files /srv/artifacts
+    mv -f *.packages /srv/artifacts
+    mv -f *.b2sum /srv/artifacts
+    mv -f *.sha256sum /srv/artifacts
+    ```
+    4) Make the .sh file executable and set the FLAVOR you want.
+    5) Tell debspawn to run the build and where to place the build result:
+    `debspawn run --external-command
+    --artifacts-out=/where/to/put/the/built/image green
+    /path/to/build-pureos.sh`
+
+    This should give you a replica of what our autobuilders did. You can
+    also take a look at the actual code in Spark:
+    https://github.com/lkorigin/laniakea-spark/blob/master/spark/runners/image_build.py#L30