Add folder for custom gitignored developer files

DEVELOPMENT.md

@@ -15,6 +15,7 @@ This comprehensive guide covers everything you need to know about developing wit
 - [Freezing to Debian Archive Snapshots](#freezing-to-debian-archive-snapshots)
 - [Testing for Reproducibility](#testing-for-reproducibility)
 - [Creating Debian Packages](#creating-debian-packages)
+- [Custom Developer Files](#custom-developer-files)
 - [Debugging and Troubleshooting](#debugging-and-troubleshooting)
 
 ## Project Structure
@@ -635,6 +636,30 @@ For systems without systemd v250+ or where Nix installation isn't feasible, you
    > Replace "btrfs" with your chosen storage driver
 5. Run the desired `mkosi` command inside the shell Podman environment
 
+## Custom Developer Files
+
+When building with the `devtools` profile, you can add your own custom files to the image without committing them to git. This is useful for adding personal SSH keys, configuration files, or debugging tools during development.
+
+### Adding Custom Files
+
+Place files in `mkosi.profiles/devtools/custom/` mirroring the filesystem structure you want:
+
+```bash
+# Add your SSH authorized keys
+mkdir -p mkosi.profiles/devtools/custom/root/.ssh
+cp ~/.ssh/id_rsa.pub mkosi.profiles/devtools/custom/root/.ssh/authorized_keys
+
+# Add a custom configuration file
+mkdir -p mkosi.profiles/devtools/custom/etc
+echo "my_setting=value" > mkosi.profiles/devtools/custom/etc/myconfig.conf
+
+# Add a debugging script
+mkdir -p mkosi.profiles/devtools/custom/usr/local/bin
+cp my-debug-script.sh mkosi.profiles/devtools/custom/usr/local/bin/
+```
+
+Files placed here will be copied into the image (like any other `ExtraTrees` directory) but will be ignored by git, so they won't be accidentally committed.
+
 ## Debugging and Troubleshooting
 
 ### mkosi Debugging

mkosi.profiles/devtools/custom/.gitignore

@@ -0,0 +1,4 @@
+# Ignore everything in this directory except .gitkeep and .gitignore
+*
+!.gitkeep
+!.gitignore

mkosi.profiles/devtools/mkosi.conf

@@ -1,5 +1,6 @@
 [Content]
 ExtraTrees=mkosi.extra
+           custom
 
 Packages=adjtimex
          apt

mkosi.profiles/devtools/mkosi.postinst

@@ -7,6 +7,9 @@ HASH=$(mkosi-chroot openssl passwd -6 -salt salt "$PASSWORD")
 mkosi-chroot passwd -u root
 mkosi-chroot usermod -p "$HASH" root
 
+# Remove git files in custom/ folder
+mkosi-chroot rm /.gitignore /.gitkeep || true
+
 if [ -f "$BUILDROOT/etc/default/dropbear" ]; then
     # Remove -s, -w, -g flags from dropbear args
     sed -i '/^DROPBEAR_EXTRA_ARGS=/s/-[swg] \?//g' "$BUILDROOT/etc/default/dropbear"