Add support for integrating into nix/nixos

modules/microsite/docs/dev.md

@@ -80,6 +80,46 @@ docspell.joex {
 }
 ```
 
+## Nix Expressions
+
+The directory `/nix` contains nix expressions to install docspell via
+the nix package manager and to integrate it into NixOS.
+
+### Testing NixOS Modules
+
+The modules can be build by building the `configuration-test.nix` file
+together with some nixpkgs version. For example:
+
+``` shell
+nixos-rebuild build-vm -I nixos-config=./configuration-test.nix \
+  -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-19.09.tar.gz
+```
+
+This will build all modules imported in `configuration-test.nix` and
+create a virtual machine containing the system. After that completes,
+the system configuration can be found behind the `./result/system`
+symlink. So it is possible to look at the generated systemd config for
+example:
+
+``` shell
+cat result/system/etc/systemd/system/docspell-joex.service
+```
+
+And with some more commands (there probably is an easier way…) the
+config file can be checked:
+
+``` shell
+cat result/system/etc/systemd/system/docspell-joex.service | grep ExecStart | cut -d'=' -f2 | xargs cat | tail -n1 | awk '{print $NF}'| sed 's/.$//' | xargs cat | jq
+```
+
+To see the module in action, the vm can be started (the first line
+sets more memory for the vm):
+
+``` shell
+export QEMU_OPTS="-m 2048"
+./result/bin/run-docspelltest-vm
+```
+
 ## ADRs
 
 Some early information about certain details can be found in the few