Add the ability to test against different Consul versions (#410)

* Modify ConsulTestcontainers to check for a CONSUL_IMAGE_VERSION
environment variable. If it exists, run tests against that Docker image.
* Make the default Consul image version "latest" so that, unless
otherwise specified, tests run against the hashicorp/consul:latest
Docker image.

src/itest/java/org/kiwiproject/consul/ConsulTestcontainers.java

@@ -1,10 +1,47 @@
 package org.kiwiproject.consul;
 
+import static java.util.Objects.isNull;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 public class ConsulTestcontainers {
 
+    private static final Logger LOG = LoggerFactory.getLogger(ConsulTestcontainers.class);
+
     private ConsulTestcontainers() {
         // utility class
     }
 
-    public static final String CONSUL_DOCKER_IMAGE_NAME = "hashicorp/consul:1.16.1";
+    /**
+     * The name of the environment variable that defines the Consul Docker image version.
+     * <p>
+     * If you want to run the tests against a specific Consul Docker image, set this
+     * environment variable to the version you want. It should match a valid tag name
+     * in the hashicorp/consul
+     * <a href="https://hub.docker.com/r/hashicorp/consul/tags">container images</a>
+     * such as {@code "latest"}, {@code "1.20"}, or a more specific version such as {@code "1.19.1"}.
+     */
+    public static final String CONSUL_DOCKER_IMAGE_VERSION_ENV_VAR = "CONSUL_IMAGE_VERSION";
+
+    /**
+     * The name of the Consul Docker container image to use, e.g., {@code "hashicorp/consul:latest"}
+     * or {@code "hashicorp/consul:1.20"}.
+     * <p>
+     * By default, this will be {@code "hashicorp/consul:latest"}. To change the value, you can
+     * set the {@code CONSUL_IMAGE_VERSION} environment variable to the <em>tag name</em> that you
+     * want to use, such as {@code "1.18.2"}. The prefix is automatically prepended to the
+     * version.
+     *
+     * @see #CONSUL_DOCKER_IMAGE_VERSION_ENV_VAR
+     */
+    public static final String CONSUL_DOCKER_IMAGE_NAME = consulImageNameFromEnvOrLatest();
+
+    private static String consulImageNameFromEnvOrLatest() {
+        var value = System.getenv(CONSUL_DOCKER_IMAGE_VERSION_ENV_VAR);
+        var consulVersion = isNull(value)  ? "latest" : value;
+        var imageName = "hashicorp/consul:" + consulVersion;
+        LOG.info("Using Consul container image: {}", imageName);
+        return imageName;
+    }
 }