Install a nightly Docker cleanup timer with init --systemd (v0.9.7)

Port of the legacy host's docker-prune.timer: 02:00 host time,
Persistent=true, docker system prune -af — but without --volumes, so
the per-repository Gradle cache volumes survive. The units are
host-global; several GitTally instances share one timer.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/test/kotlin/de/hoennig/gittally/commands/SystemdServiceFilesTest.kt

@@ -3,6 +3,7 @@ package de.hoennig.gittally.commands
 import io.kotest.core.spec.style.FunSpec
 import io.kotest.matchers.shouldBe
 import io.kotest.matchers.string.shouldContain
+import io.kotest.matchers.string.shouldNotContain
 import java.nio.file.Paths
 
 class SystemdServiceFilesTest : FunSpec() {
@@ -46,6 +47,24 @@ class SystemdServiceFilesTest : FunSpec() {
             content shouldContain """-jar "/srv/100%%-repo/gittally.jar" server"""
         }
 
+        test("prune service cleans containers and images but never volumes") {
+            val content = SystemdServiceFiles.pruneServiceContent()
+
+            content shouldContain "Type=oneshot"
+            content shouldContain "ExecStart=docker system prune -af"
+            // the per-repository Gradle cache volumes must survive the cleanup
+            content shouldNotContain "--volumes"
+            content shouldContain "ExecCondition=sh -c 'command -v docker'"
+        }
+
+        test("prune timer fires nightly at 02:00 and catches up after downtime") {
+            val content = SystemdServiceFiles.pruneTimerContent()
+
+            content shouldContain "OnCalendar=*-*-* 02:00:00"
+            content shouldContain "Persistent=true"
+            content shouldContain "WantedBy=timers.target"
+        }
+
         test("environment file template only tunes the JVM") {
             val content = SystemdServiceFiles.envFileContent()