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/Dockerfiles/Whiteboard.j2

@@ -0,0 +1,27 @@
+FROM {{ NEXTCLOUD_WHITEBOARD_IMAGE }}:{{ NEXTCLOUD_WHITEBOARD_VERSION }}
+
+# Temporarily switch to root so we can install packages
+USER 0
+
+# Install Chromium, ffmpeg, fonts, and runtime libraries for headless operation on Alpine
+RUN apk add --no-cache \
+      chromium \
+      ffmpeg \
+      nss \
+      freetype \
+      harfbuzz \
+      ttf-dejavu \
+      ttf-liberation \
+      udev \
+      ca-certificates \
+    && update-ca-certificates
+
+# Ensure a consistent Chromium binary path
+RUN if [ -x /usr/bin/chromium-browser ]; then ln -sf /usr/bin/chromium-browser /usr/bin/chromium; fi
+
+# Environment variables used by Puppeteer
+ENV PUPPETEER_EXECUTABLE_PATH=/usr/bin/chromium \
+    PUPPETEER_SKIP_DOWNLOAD=true
+
+# Switch back to the original non-root user (nobody)
+USER 65534