README: add building steps

README.md

@@ -18,7 +18,77 @@ Thus the following use cases are still not supported / tested:
  * `qubes-vm-kernel-support` Not adapted for use on Alpine yet, due to it providing a Dracut module. In most cases, it is not necessary as Qubes provides the kernel. 
    This package is only neccessary when VM uses its own kernel, thus a hook is added to Dracut to generate the initrd for use within qubes.
 
-#### Issues, recommendations and proposals
+## How to build
+
+Since this template builder uses pre-built Alpine Linux packages for QubesOS template support, it by defaults does not build everything locally. If you want to build them locally, you can follow these steps. If not, you can skip to the next section.
+
+### Build packages
+
+**1) Set-up build environment on Alpine Linux**
+```
+apk add alpine-sdk lua-aports
+addgroup <yourusername> abuild
+mkdir -p /var/cache/distfiles
+chmod a+w /var/cache/distfiles
+abuild-keygen -a -i
+cp /home/user/.abuild/*.pub /etc/apk/keys
+```
+(see [Creating an Alpine package](https://wiki.alpinelinux.org/wiki/Creating_an_Alpine_package) on Alpine Linux Wiki for more details)
+
+**2) Create work directory and clone `qports` with appropriate branch**
+```
+mkdir work
+cd work
+git clone https://ayakael.net/forge/qports -b <desired Qubes Rel, i.e r4.2>
+```
+
+**3) Build all packages using `buildrepo`**
+`buildrepo -a <path/to/qports> -d <path/to/work/dir>/packages qports` 
+
+**4) Make these packages available on an HTTP server and export the following variables**
+```
+export QUBESALPINE_MIRROR=<https://url-to-custom-repo>
+export QUBESALPINE_KEYFILE=<https://url-to-custom-repo-key>
+```
+
+### Build template
+
+**1) Create work directory and clone `qubes-builder-alpine` with appropriate branch**
+```
+mkdir work
+cd work
+git clone https://ayakael.net/forge/qubes-builder-alpine -b <desred Qubes Rel, i.e r4.2>
+```
+
+**2) Set-up build environment for QubesOS template build**
+```
+git clone https://github.com/QubesOS/qubes-builder
+mkdir qubes-builder/qubes-src
+ln -s $(pwd) qubes-builder/qubes-src/builder-alpine
+cp builder.conf qubes-builder/.
+git clone https://github.com/QubesOS/qubes-linux-template-builder qubes-builder/qubes-src/linux-template-builder
+```
+
+**3) Setup build information**
+```
+echo "%define _arch x86_64" >> qubes-builder/qubes-src/linux-template-builder/templates.spec
+echo "4.2.0" > qubes-builder/qubes-src/linux-template-builder/version
+```
+If you want to build a different Alpine version than default, you can modify `qubes-builder/builder.conf` on line `DIST_VM` to alpine<desired-version-without-dot> (i.e alpine320).
+
+**4) Build template**
+```
+cd qubes-builder
+make linux-template-builder
+```
+
+**5) Install produced RPM on dom0**
+```
+qvm-run --pass-io <build-vm> 'cat <path/to/work/dir>/qubes-builder/qubes-src/linux-template-builder/rpm/noarch/qubes-template-*.rpm' > qubes-template-alpine.rpm
+qvm-template install --nogpgcheck $(pwd)/qubes-template-alpine.rpm
+```
+
+## Issues, recommendations and proposals
 **To report an issue or share a recommendation**
 
 Go [here](https://gitlab.alpinelinux.org/ayakael/qubes-builder-alpine/-/issues)