fix: Add --network=host to single-arch Docker builds

Fixes network isolation issue in Docker builds where containers couldn't
reach external networks (PyPI, apt repos, etc).

**Problem:**
- Multi-arch buildx builds use: --driver-opt network=host ✅
- Single-arch builds were missing: --network=host ❌
- Result: Docker build containers had [Errno 101] Network unreachable

**Root Cause:**
When docker build runs without --network=host, build containers are
isolated from the host network and can't reach external services.

**Solution:**
Add --network=host flag to single-arch docker build command on line 131.

**Testing:**
This matches the configuration used successfully in multi-arch builds.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

first-class-pipeline/src/com/freeleaps/devops/ImageBuilder.groovy

@@ -128,7 +128,7 @@ class ImageBuilder {
               architectures.each { architecture ->
                 def archTag = architecture.split("/")[1]
                 steps.log.info("ImageBuilder", "Building image ${registry}/${repository}/${name} with architectures: ${architectures}, tag sets to ${version}")
-                steps.sh "docker build -t ${registry}/${repository}/${name}:${version} --platform ${architecture} -f ${dockerfile} ${contextRoot}"
+                steps.sh "docker build --network=host -t ${registry}/${repository}/${name}:${version} --platform ${architecture} -f ${dockerfile} ${contextRoot}"
                 steps.sh "docker push ${registry}/${repository}/${name}:${version}"
               }
               steps.env.BUILD_IMAGE_REGISTRY = "${registry}"