JKNS-458: Add Konflux-compatible Dockerfile

Adding a Dockerfile for testing the Jenkins client plugin that does not
rely on the OpenShift CI registry for base images. This will pay
dividends when we eventually build the Jenkins images and plugins on
Konflux, as this Dockerfile only uses productized images published on
registry.redhat.io.

This also adds a .dockerignore to ensure generated or test content is
not present when developers build this image themselves locally.

Signed-off-by: Adam Kaplan <adam.kaplan@redhat.com>

.dockerignore

@@ -0,0 +1,9 @@
+.idea
+target
+PR-Testing/jpi/
+openshift-client.iml
+.classpath
+.project
+.settings
+.DS_Store
+.vscode

.gitignore

@@ -6,3 +6,4 @@ openshift-client.iml
 .project
 .settings
 .DS_Store
+.vscode

Dockerfile.rhel8

@@ -0,0 +1,19 @@
+# This Dockerfile creates a test image that builds the openshift-client plugin and installs it into
+# the latest Jenkins image, replacing the official relased version of the OCP client plugin.
+# The image is intended to be used by the CI system to test changes.
+
+FROM registry.redhat.io/ubi8/openjdk-17:1.20 AS builder
+WORKDIR /java/src/github.com/openshift/jenkins-client-plugin
+# openjdk-17 image defaults the USER to jboss. Use --chown flag to set the owner of the copied
+# files to jboss. Otherwise the files will be owned by root and the maven build will fail.
+COPY --chown=jboss:jboss . .
+RUN mvn --version && \
+    mvn clean package
+
+# Use the latest Jenkins image as the base for the "runtime" image.
+# The tag should be updated to reflect the appropriate OCP version being tested - often this
+# will lag the current version of OCP under development.
+FROM registry.redhat.io/ocp-tools-4/jenkins-rhel8:v4.14.0
+RUN rm /opt/openshift/plugins/openshift-client.jpi
+COPY --from=builder /java/src/github.com/openshift/jenkins-client-plugin/target/openshift-client.hpi /opt/openshift/plugins
+RUN mv /opt/openshift/plugins/openshift-client.hpi /opt/openshift/plugins/openshift-client.jpi