build/ci: install headless JDK

As discussed in monogon-dev/monogon#83, we can avoid
pulling in a number of unnecessary dependencies by using
a headless JDK. Improves upon I4622349, which would break Jenkins.

Change-Id: Iff4aef30b9c1ae76293d9a29cf0a86945eabc679
Reviewed-on: https://review.monogon.dev/c/monogon/+/414
Reviewed-by: Sergiusz Bazanski <serge@monogon.tech>
diff --git a/build/ci/Dockerfile b/build/ci/Dockerfile
index 5f5895c..a92824c 100644
--- a/build/ci/Dockerfile
+++ b/build/ci/Dockerfile
@@ -30,7 +30,12 @@
 	grpc-cli \
 	nc \
 	python-unversioned-command \
-	openssl-devel
+	openssl-devel \
+	java-11-openjdk-headless
+
+
+# (java-11-openjdk-headless is required for the Jenkins CI agent)
+
 
 # Create CI build user. This is not used by scripts/bin/bazel, but instead only
 # used by CI infrastructure to run build agents as.