Document container creation steps (#12)

Author: rvansa

Dockerfile.checkpoint

@@ -0,0 +1,23 @@
+ARG JDK_NAME=zulu22.30.13-ca-crac-jdk22.0.1-linux_x64
+
+FROM ubuntu:24.04 as builder
+ARG JDK_NAME
+
+RUN apt-get update && apt-get install -y wget
+RUN wget -O crac-jdk.tar.gz https://cdn.azul.com/zulu/bin/$JDK_NAME.tar.gz
+RUN tar zxf ./crac-jdk.tar.gz -C /usr/share
+
+# End of builder
+
+FROM ubuntu:24.04
+ARG JDK_NAME
+ARG FAT_JAR=
+
+ENV JDK_NAME=$JDK_NAME
+ENV JAVA_HOME=/usr/share/$JDK_NAME
+
+COPY --from=builder /usr/share/${JDK_NAME} /usr/share/${JDK_NAME}
+RUN ln -s $JAVA_HOME/bin/java /bin/ && ln -s $JAVA_HOME/bin/jcmd /bin/
+ADD target/example-spring-boot*.jar  /example-spring-boot.jar
+ENTRYPOINT [ "java", "-XX:CPUFeatures=generic", "-XX:CRaCCheckpointTo=/cr", "-jar", "/example-spring-boot.jar" ]
+

Dockerfile.restore

@@ -0,0 +1,5 @@
+FROM example-spring-boot-checkpoint
+ADD target/cr /cr
+ENTRYPOINT [ "java", "-XX:CRaCRestoreFrom=/cr" ]
+
+

README.md

@@ -38,3 +38,38 @@ jcmd target/example-spring-boot-0.0.1-SNAPSHOT.jar JDK.checkpoint
 ```
 $JAVA_HOME/bin/java -XX:CRaCRestoreFrom=cr
 ```
+
+## Preparing a container image
+
+1. After building the project locally create an image to be checkpointed.
+```
+docker build -f Dockerfile.checkpoint -t example-spring-boot-checkpoint .
+```
+
+2.  Start a (detached) container that will be checkpointed. Note that we're mounting `target/cr` into the container.
+```
+docker run -d --rm -v $(pwd)/target/cr:/cr --cap-add=CHECKPOINT_RESTORE --cap-add=SYS_PTRACE -p 8080:8080 --name example-spring-boot-checkpoint example-spring-boot-checkpoint
+```
+
+3. Validate that the container is up and running (here you could also perform any warm-up)
+```
+curl localhost:8080
+Greetings from Spring Boot!
+```
+
+4. Checkpoint the running container
+```
+docker exec -it example-spring-boot-checkpoint jcmd example-spring-boot JDK.checkpoint
+```
+
+5. Build another container image by adding the data from `target/cr` on top of the previous image and adjusting entrypoint:
+```
+docker build -f Dockerfile.restore -t example-spring-boot .
+```
+
+6. (Optional) Start the application in the restored container and validate that it works
+```
+docker run -it --rm -p 8080:8080 example-spring-boot
+# In another terminal
+curl localhost:8080
+```