build/ci/Dockerfile: add tini package

See Idaa0b856 for the internal infra change.

Change-Id: Ic699b8298204e6e4d1bc68993ea377b4054cc80e
Reviewed-on: https://review.monogon.dev/c/monogon/+/696
Reviewed-by: Sergiusz Bazanski <serge@monogon.tech>
diff --git a/build/ci/Dockerfile b/build/ci/Dockerfile
index 81101de..f2aef95 100644
--- a/build/ci/Dockerfile
+++ b/build/ci/Dockerfile
@@ -32,10 +32,11 @@
 	nc \
 	python-unversioned-command \
 	openssl-devel \
-	java-11-openjdk-headless
+	java-11-openjdk-headless \
+	tini
 
 
-# (java-11-openjdk-headless is required for the Jenkins CI agent)
+# (java-11-openjdk-headless and tini are required for the Jenkins CI agent)
 
 
 # Create CI build user. This is not used by scripts/bin/bazel, but instead only