Add Playground Apps Manager - Web-based devcontainer app deployment system

src/playground/.devcontainer.json

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+{
+  "name": "Playground Apps Manager",
+  "dockerComposeFile": "docker-compose.yaml",
+  "service": "app",
+  "shutdownAction": "none",
+  "workspaceFolder": "/workspace",
+  "initializeCommand": "DOCKER_GID=`getent group docker | cut -d: -f3` && echo \"DOCKER_GID=${DOCKER_GID}\" > .env && mkdir -p apps && chmod 777 apps",
+  "remoteUser": "root"
+}

src/playground/README.md

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+# example
+
+Web-based manager for creating and deploying custom devcontainer applications
+
+## Access
+
+Once deployed in Workbench, access the web interface at the app URL (port 8080).
+
+For local testing:
+1. Create Docker network: `docker network create app-network`
+2. Run the app: `devcontainer up --workspace-folder .`
+3. Access at: `http://localhost:8080`
+
+## Customization
+
+Edit the following files to customize your app:
+
+- `.devcontainer.json` - Devcontainer configuration and features
+- `docker-compose.yaml` - Docker Compose configuration (change the `command` to customize ttyd options)
+- `devcontainer-template.json` - Template options and metadata
+
+## Usage
+
+1. Fork the repository
+2. Modify the configuration files as needed
+3. In Workbench UI, create a custom app pointing to your forked repository
+4. Select this app template (example)

src/playground/caddy/Caddyfile

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+{
+    debug
+    admin 0.0.0.0:2019
+}
+
+http://:8080 {
+    # Dynamic routes will be added here via API
+    route {
+        # Default route to playground UI
+        reverse_proxy playground:8080
+    }
+}

src/playground/devcontainer-template.json

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+{
+  "id": "playground",
+  "version": "1.0.0",
+  "name": "Playground Apps Manager",
+  "description": "Web-based manager for creating and deploying custom devcontainer applications"
+}

src/playground/docker-compose.yaml

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+services:
+  app:
+    # The container name must be "application-server"
+    container_name: "application-server"
+    image: "caddy:2.11-alpine"
+    restart: always
+    volumes:
+      - ./caddy:/etc/caddy:ro
+      - caddy_data:/data
+    ports:
+      - 8080:8080
+    networks:
+      - app-network
+      - playground-apps
+
+  playground:
+    build:
+      context: ./playground
+    container_name: "playground"
+    user: "playground:${DOCKER_GID}"
+    restart: always
+    depends_on:
+      app:
+        condition: service_started
+      db:
+        condition: service_healthy
+    environment:
+      PORT: "8080"
+      DB_HOST: "db"
+      DB_PORT: "5432"
+      DB_NAME: "playground"
+      DB_USER: "playground_user"
+      DB_PASSWORD: "playground_password"
+      CADDY_HOST: "application-server"
+      CADDY_PORT: "2019"
+    volumes:
+      # mount Host machine's docker.sock to container's docker.sock
+      - /var/run/docker.sock:/var/run/docker.sock
+      # mount Host machine's /etc/group to container's /etc/host-group
+      - /etc/group:/etc/host-group
+      # mount apps directory for devcontainer builds
+      - ./apps:/workspace/apps
+      # mount startup scripts from parent directory
+      - ./startupscript:/workspace/startupscript
+      # mount features directory for devcontainer features
+      - ../../features:/workspace/features:ro
+    networks:
+      - playground-apps
+      - playground
+
+  db:
+    image: "postgres:18-alpine"
+    restart: always
+    environment:
+      POSTGRES_USER: "playground_user"
+      POSTGRES_PASSWORD: "playground_password"
+      POSTGRES_DB: "playground"
+    volumes:
+      - pg_data:/var/lib/postgresql/data
+    networks:
+      - playground
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U $${POSTGRES_USER} -d $${POSTGRES_DB}"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+      start_period: 10s
+
+volumes:
+  caddy_data:
+  pg_data:
+
+networks:
+  # The Docker network must be named "app-network". This is an external network
+  # that is created outside of this docker-compose file.
+  app-network:
+    external: true
+  playground:
+  playground-apps:

src/playground/playground/Dockerfile

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+FROM golang:1.25-alpine AS build
+
+WORKDIR /app
+
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+
+RUN go build -o /playground .
+
+FROM alpine:latest
+
+# Install Node.js, npm, Docker CLI, docker-compose, buildx, ttyd, bash, and devcontainer CLI
+RUN apk add --no-cache nodejs npm docker-cli docker-cli-compose docker-cli-buildx ttyd bash && \
+    npm install -g @devcontainers/cli
+
+RUN addgroup -S playground -g 1001 && adduser -S playground -u 1001 -G playground
+
+WORKDIR /workspace
+COPY --chown=playground:playground --from=build /playground /playground
+
+# Create startup script to run ttyd in background and playground in foreground
+RUN cat > /start.sh <<'EOF'
+#!/bin/sh
+ttyd --writable --port 7681 --base-path /_shell bash &
+exec /playground
+EOF
+
+RUN chmod +x /start.sh
+
+USER playground
+
+EXPOSE 8080 7681
+
+CMD [ "/start.sh" ]