implemented 11-docker-build-runtime.md: added optional Docker-based build execution with configuration, per-branch runtime selection, image rebuild on input changes, Gradle cache volume, and ownership repair; updated docs and configuration

src/main/kotlin/de/hoennig/gittally/build/BuildRunner.kt

@@ -1,18 +1,23 @@
 package de.hoennig.gittally.build
 
+import de.hoennig.gittally.config.BranchConfig
+import org.springframework.context.annotation.Primary
 import org.springframework.stereotype.Component
 import java.nio.file.Path
 
 /**
  * Starts a single build or clean command and hands the [Process] back to the caller,
  * which owns log streaming and process-tree termination.
- * Native shell execution for now; a Docker runner can plug in later (step 11).
+ * [repoDir] and [branchConfig] let implementations derive per-repository resources
+ * and per-branch settings (step 11: Docker runtime).
  */
 interface BuildRunner {
     fun start(
         command: String,
         workingDir: Path,
         environment: Map<String, String>,
+        repoDir: Path = workingDir,
+        branchConfig: BranchConfig = BranchConfig(),
     ): Process
 }
 
@@ -22,6 +27,8 @@ class ProcessBuildRunner : BuildRunner {
         command: String,
         workingDir: Path,
         environment: Map<String, String>,
+        repoDir: Path,
+        branchConfig: BranchConfig,
     ): Process {
         val processBuilder = ProcessBuilder("bash", "-c", command)
         processBuilder.directory(workingDir.toFile())
@@ -29,3 +36,25 @@ class ProcessBuildRunner : BuildRunner {
         return processBuilder.start()
     }
 }
+
+/**
+ * Selects the runtime per branch: Docker when `branches.<name>.docker.enabled`,
+ * native shell execution otherwise (the unchanged default).
+ */
+@Primary
+@Component
+class DispatchingBuildRunner(
+    private val processBuildRunner: ProcessBuildRunner,
+    private val dockerBuildRunner: DockerBuildRunner,
+) : BuildRunner {
+    override fun start(
+        command: String,
+        workingDir: Path,
+        environment: Map<String, String>,
+        repoDir: Path,
+        branchConfig: BranchConfig,
+    ): Process {
+        val runner = if (branchConfig.docker.enabled) dockerBuildRunner else processBuildRunner
+        return runner.start(command, workingDir, environment, repoDir, branchConfig)
+    }
+}