Ubuntu2204: Install openssh-client package (#1818)

Related: #1803.

Some downstream projects depend on openssh-client, most notably Gerrit
Code Review.

While it could be an option to allow custom docker images for downstream
projects, or even install additional packages dynamically during the
build, this change follows a pragmatic approach, to just pre-install the
openssh-client package in the base container image.
diff --git a/buildkite/docker/ubuntu2204/Dockerfile b/buildkite/docker/ubuntu2204/Dockerfile
index 6b98beb..37cdc35 100644
--- a/buildkite/docker/ubuntu2204/Dockerfile
+++ b/buildkite/docker/ubuntu2204/Dockerfile
@@ -33,6 +33,7 @@
     lsb-release \
     netcat-openbsd \
     openjdk-17-jdk-headless \
+    openssh-client \
     python-is-python3 \
     python2 \
     python2-dev \