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/DockerImageInputs.kt

@@ -0,0 +1,28 @@
+package de.hoennig.gittally.build
+
+import java.nio.file.Files
+import java.nio.file.Path
+import java.security.MessageDigest
+
+/**
+ * Staleness checksum of the Docker image build inputs, compatible with legacy
+ * `docker_build_inputs_sha256`: the SHA-256 of the Dockerfile contents combined
+ * with the configured Dockerfile and context paths, stored as an image label.
+ */
+object DockerImageInputs {
+    const val INPUTS_LABEL = "org.gittally.build-inputs-sha256"
+
+    fun dockerfileSha256(dockerfile: Path): String = sha256Hex(Files.readAllBytes(dockerfile))
+
+    fun inputsSha256(
+        dockerfileHash: String,
+        dockerfile: String,
+        context: String,
+    ): String = sha256Hex("$dockerfileHash\n$dockerfile\n$context\n".toByteArray())
+
+    private fun sha256Hex(bytes: ByteArray): String =
+        MessageDigest
+            .getInstance("SHA-256")
+            .digest(bytes)
+            .joinToString("") { "%02x".format(it) }
+}