test: add test images build instructions and stacker.yamls (#2249)

Signed-off-by: Andrei Aaron <aaaron@luxoft.com>

test/images/README.md

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+# Build scripts for images used in tests
+
+## General instructions
+
+This folder contains build files used to produce oci images used in zot tests.
+Build them using stacker, and copy them over to ghrc.io using skopeo.
+
+For example in case of the java image:
+
+```bash
+stacker build -f stacker-java.yaml
+```
+
+Check the image is scanned correctly using a trivy binary, in order to make sure it does
+or does not contain expected vulnerabilities, in case the image is to be used for CVE scanning.
+
+```bash
+trivy image scan --input oci:java-test
+```
+
+Copy the new image over to ghcr.io using skopeo
+
+```bash
+skopeo copy --dest-creds=<user>:<token> oci:oci:java-test docker://ghcr.io/project-zot/test-images/java:0.0.1
+```
+
+## Images
+
+### Java
+
+The file stacker-java.yaml is used to produce the images in the repo at: ghcr.io/project-zot/test-images/java
+Basically we compile a simple java file and without any vulnerabilities.
+We can test the CVE scanning of Java images, including zot downloading the Java vulnerability DB.
+
+### Spring
+
+The file stacker-spring.yaml is used to produce the images in the repo at: ghcr.io/project-zot/test-images/spring-web
+We just copy and download the already compiled spring jar file.
+It can be scanned to identify at least one Java specific vulnerability in zot tests.

test/images/stacker-java.yaml

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+build-java-test:
+  from:
+    type: docker
+    url: docker://zothub.io/c3/ubuntu/openj9-devel-amd64:11
+  import:
+    - test.java
+  run: |
+    . /etc/profile
+    env # export PATH=$PATH:/bin:/usr/bin
+    javac -version
+    java -version
+    mkdir /workspace
+    cp /stacker/test.java /workspace/
+    cd /workspace/
+    mkdir META-INF
+    echo "Main-Class: test" > META-INF/MANIFEST.MF
+    javac test.java
+    ls
+    java test
+    jar cmvf META-INF/MANIFEST.MF test.jar test.class
+    ls
+    jar tf test.jar
+    java -jar test.jar
+  build_only: true
+
+java-test:
+  from:
+    type: scratch
+  import:
+    path: stacker://build-java-test/workspace/
+    dest: /

test/images/stacker-spring.yaml

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+spring-test:
+  from:
+    type: scratch
+  import:
+    - path: "https://repo1.maven.org/maven2/org/springframework/spring-web/5.3.31/spring-web-5.3.31.jar"
+      dest: /usr/local/artifacts/

test/images/test.java

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+public class test {
+  public static void main(String[] args) {
+    System.out.println("hello world!");
+  }
+}