fix: Dockerfile → COPY pre-built binary (no network needed)

Docker Desktop Windows blocks build network access. Instead of
building V from source inside Docker, cross-compile Linux binary
on host and COPY into minimal alpine image.

docker_install.v

@@ -50,24 +50,20 @@ state:
 		println('  Wrote: ${target_dir}/dashboard.html')
 	}
 
-	// Copy Dockerfile and sources for build
+	// Copy Dockerfile and build Linux binary
 	if os.exists('Dockerfile') {
 		os.cp('Dockerfile', '${target_dir}/Dockerfile') or {}
 		println('  Wrote: ${target_dir}/Dockerfile')
 	}
-	// Copy V source files
-	src_files := ['main.v', 'server.v', 'config.v', 'log.v', 'ntfy.v', 'dedup.v',
-		'docker_watcher.v', 'service.v', 'filter_outgoing.v', 'metrics.v',
-		'docker_install.v', 'v.mod', 'dashboard.html', 'ntfy-bridge.example.yaml',
-		'sources/event.v', 'sources/gitea.v', 'sources/uptime_kuma.v',
-		'sources/docker.v', 'sources/generic.v', 'sources/cron.v',
-		'sources/http_poll.v']
-	for file in src_files {
-		if os.exists(file) {
-			target := '${target_dir}/${file}'
-			os.mkdir_all(os.dir(target)) or {}
-			os.cp(file, target) or {}
-		}
+	// Cross-compile Linux binary
+	println('  Building Linux binary (v -os linux)...')
+	build_result := os.execute('v -os linux -o ${target_dir}/ntfy-bridge .')
+	if build_result.exit_code != 0 {
+		println('  Warning: Could not cross-compile Linux binary.')
+		println('  ${build_result.output}')
+		println('  You will need a Linux binary named "ntfy-bridge" in ${target_dir}/')
+	} else {
+		println('  Built: ${target_dir}/ntfy-bridge')
 	}
 
 	// docker-compose.yml