feat(nextcloud): enable custom Alpine-based Whiteboard image with Chromium & ffmpeg support

- Added role tasks to deploy templated Dockerfile for Whiteboard service
- Configured build context and custom image name (nextcloud_whiteboard_custom)
- Increased PID limits and shm_size for stable recording
- Adjusted user ID variable naming consistency
- Integrated path_join for service directory variables
- Fixed build permissions (install as root, revert to nobody)

Reference: ChatGPT conversation https://chatgpt.com/share/68f771c6-0e98-800f-99ca-9e367f4cd0c2

roles/web-app-nextcloud/templates/env.j2

@@ -65,6 +65,4 @@ CHROMIUM_FLAGS=--headless=new --no-sandbox --disable-gpu --disable-dev-shm-usage
 # Falls das Image Chromium mitbringt – Pfad meistens /usr/bin/chromium oder /usr/bin/chromium-browser:
 PUPPETEER_EXECUTABLE_PATH=/usr/bin/chromium
 PUPPETEER_SKIP_DOWNLOAD=true
-# Deactivated @todo implement
-WHITEBOARD_ENABLE_RECORDING=false
 {% endif %}