[a2a] Do not build test python server image if no docker installed

Author: EugeneTheDev

a2a/a2a-client/build.gradle.kts

@@ -1,4 +1,6 @@
 import ai.koog.gradle.publish.maven.Publishing.publishToMaven
+import org.gradle.kotlin.dsl.support.serviceOf
+import java.io.ByteArrayOutputStream
 
 group = rootProject.group
 version = rootProject.version
@@ -59,6 +61,23 @@ tasks.register<Exec>("dockerBuildTestPythonA2AServer") {
     description = "Build Python A2A test server image"
     workingDir = file("../test-python-a2a-server")
     commandLine = listOf("docker", "build", "-t", "test-python-a2a-server", ".")
+
+    onlyIf {
+        val buffer = ByteArrayOutputStream()
+        val result = serviceOf<ExecOperations>().exec {
+            commandLine = listOf("docker", "--version")
+            isIgnoreExitValue = true
+            standardOutput = buffer
+            errorOutput = buffer
+        }
+
+        val available = result.exitValue == 0
+        if (!available) {
+            logger.warn("Docker not found. Skipping task 'dockerBuildTestPythonA2AServer'")
+        }
+
+        available
+    }
 }
 
 tasks.named("jvmTest") {