Add Docker CLI support to Docker image with INSTALL_EXT build argument (#366)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: samanhappy <2755122+samanhappy@users.noreply.github.com>

.github/DOCKER_CLI_TEST.md

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+# Docker CLI Installation Test Procedure
+
+This document describes how to test the Docker CLI installation feature added with the `INSTALL_EXT=true` build argument.
+
+## Test 1: Build with INSTALL_EXT=false (default)
+
+```bash
+# Build without extended features
+docker build -t mcphub:base .
+
+# Run the container
+docker run --rm mcphub:base docker --version
+```
+
+**Expected Result**: `docker: not found` error (Docker CLI is NOT installed)
+
+## Test 2: Build with INSTALL_EXT=true
+
+```bash
+# Build with extended features
+docker build --build-arg INSTALL_EXT=true -t mcphub:extended .
+
+# Test Docker CLI is available
+docker run --rm mcphub:extended docker --version
+```
+
+**Expected Result**: Docker version output (e.g., `Docker version 27.x.x, build xxxxx`)
+
+## Test 3: Docker-in-Docker Workflow
+
+```bash
+# Build with extended features
+docker build --build-arg INSTALL_EXT=true -t mcphub:extended .
+
+# Run with Docker socket mounted
+docker run -d \
+  --name mcphub-test \
+  -p 3000:3000 \
+  -v /var/run/docker.sock:/var/run/docker.sock \
+  mcphub:extended
+
+# Test Docker commands from inside the container
+docker exec mcphub-test docker ps
+docker exec mcphub-test docker images
+
+# Cleanup
+docker stop mcphub-test
+docker rm mcphub-test
+```
+
+**Expected Result**: Docker commands should work and show the host's containers and images
+
+## Test 4: Verify Image Size
+
+```bash
+# Build both versions
+docker build -t mcphub:base .
+docker build --build-arg INSTALL_EXT=true -t mcphub:extended .
+
+# Compare image sizes
+docker images mcphub:*
+```
+
+**Expected Result**: 
+- The `extended` image should be larger than the `base` image
+- The size difference should be reasonable (Docker CLI adds ~60-80MB)
+
+## Test 5: Architecture Support
+
+```bash
+# On AMD64/x86_64
+docker build --build-arg INSTALL_EXT=true --platform linux/amd64 -t mcphub:extended-amd64 .
+
+# On ARM64
+docker build --build-arg INSTALL_EXT=true --platform linux/arm64 -t mcphub:extended-arm64 .
+```
+
+**Expected Result**: 
+- Both builds should succeed
+- AMD64 includes Chrome/Playwright + Docker CLI
+- ARM64 includes Docker CLI only (Chrome installation is skipped)
+
+## Notes
+
+- The Docker CLI installation follows the official Docker documentation
+- The installation uses the Debian Bookworm repository
+- All temporary files are cleaned up to minimize image size
+- The feature is opt-in via the `INSTALL_EXT` build argument