feat: startingpoint modules (#33)

* feat: add startingpoint modules
some modules authored by @gerblesh
original source: https://github.com/ublue-os/startingpoint/pull/135

* docs: better readme for modules dir

* docs: yafti deps

* docs: sp more information

* feat: startingpoint modules in container

* docs: sentence structure, check sidebar

modules/script/README.md

@@ -0,0 +1,26 @@
+# `script` Module for Startingpoint
+
+The `script` module can be used to run arbitrary scripts at image build time that take no or minimal external configuration (in the form of command line arguments).
+The scripts, which are run from the `config/scripts` directory, are declared under `scripts:`.
+
+## Example Configuration
+
+```yml
+type: script
+scripts:
+    - signing.sh 
+```
+
+## Creating a Script
+
+Look at `example.sh` for an example shell script. You can rename and copy the file for your own purposes. In order for the script to be executed, declare it in the recipe
+
+When creating a script, please make sure
+
+- ...its filename ends with `.sh`.
+  - This follows convention for (especially bash) shell scripts.
+  - `autorun.sh` only executes files that match `*.sh`.
+- ...it starts with a [shebang](<https://en.wikipedia.org/wiki/Shebang_(Unix)>) like `#!/usr/bin/env bash`.
+  - This ensures the script is ran with the correct interpreter / shell.
+- ...it contains the command `set -oue pipefail` near the start.
+  - This will make the image build fail if your script fails. If you do not care if your script works or not, you can omit this line.