Add utility class to allow image names to be configured via a classpa…

…th file

Replace static string names for ambassador and vnc recorder container images with classpath lookup.

Add note to readme re overriding hardcoded image names

Refs #259, #276

core/src/main/java/org/testcontainers/containers/AmbassadorContainer.java

@@ -5,6 +5,7 @@
 import lombok.Data;
 import lombok.EqualsAndHashCode;
 import org.testcontainers.containers.traits.LinkableContainer;
+import org.testcontainers.utility.TestcontainersConfiguration;
 
 import java.util.List;
 
@@ -24,7 +25,7 @@ public class AmbassadorContainer<SELF extends AmbassadorContainer<SELF>> extends
     private final int servicePort;
 
     public AmbassadorContainer(LinkableContainer otherContainer, String serviceName, int servicePort) {
-        super("richnorth/ambassador:latest");
+        super(TestcontainersConfiguration.getInstance().ambassadorContainerImage);
 
         /*
           Use the unique 'identifierPrefix' (random compose project name) so that the ambassador can see

core/src/main/java/org/testcontainers/containers/VncRecordingSidekickContainer.java

@@ -3,6 +3,7 @@
 import com.github.dockerjava.api.command.InspectContainerResponse;
 import org.testcontainers.containers.traits.LinkableContainer;
 import org.testcontainers.containers.traits.VncService;
+import org.testcontainers.utility.TestcontainersConfiguration;
 
 import java.io.File;
 import java.io.IOException;
@@ -23,7 +24,7 @@ public class VncRecordingSidekickContainer<SELF extends VncRecordingSidekickCont
      * @param vncServiceContainer the container whose screen should be recorded. This container must implement VncService and LinkableContainer.
      */
     public VncRecordingSidekickContainer(T vncServiceContainer) {
-        super("richnorth/vnc-recorder:latest");
+        super(TestcontainersConfiguration.getInstance().vncRecordedContainerImage);
 
         this.vncServiceContainer = vncServiceContainer;
 

core/src/main/java/org/testcontainers/utility/TestcontainersConfiguration.java

@@ -0,0 +1,57 @@
+package org.testcontainers.utility;
+
+import com.google.common.base.MoreObjects;
+import lombok.Data;
+import lombok.Getter;
+import lombok.extern.slf4j.Slf4j;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.net.URL;
+import java.util.Properties;
+
+/**
+ * Provides a mechanism for fetching configuration/defaults from the classpath.
+ */
+@Data
+@Slf4j
+public class TestcontainersConfiguration {
+
+    // No public constructor - must use getInstance() method.
+    private TestcontainersConfiguration() {
+    }
+
+    @Getter(lazy = true)
+    private static final TestcontainersConfiguration instance = loadConfiguration();
+
+    private static TestcontainersConfiguration loadConfiguration() {
+        final TestcontainersConfiguration config = new TestcontainersConfiguration();
+
+        ClassLoader loader = MoreObjects.firstNonNull(
+                Thread.currentThread().getContextClassLoader(),
+                TestcontainersConfiguration.class.getClassLoader());
+        final URL configOverrides = loader.getResource("testcontainers.properties");
+        if (configOverrides != null) {
+
+            log.debug("Testcontainers configuration overrides will be loaded from {}", configOverrides);
+
+            final Properties properties = new Properties();
+            try (final InputStream inputStream = configOverrides.openStream()) {
+                properties.load(inputStream);
+
+                config.ambassadorContainerImage = properties.getProperty("ambassador.container.image", config.ambassadorContainerImage);
+                config.vncRecordedContainerImage = properties.getProperty("vncrecorder.container.image", config.vncRecordedContainerImage);
+
+                log.debug("Testcontainers configuration overrides loaded from {}: {}", configOverrides, config);
+
+            } catch (IOException e) {
+                log.error("Testcontainers config override was found on classpath but could not be loaded", e);
+            }
+        }
+
+        return config;
+    }
+
+    public String ambassadorContainerImage = "richnorth/ambassador:latest";
+    public String vncRecordedContainerImage = "richnorth/vnc-recorder:latest";
+}

docs/usage.md

@@ -97,4 +97,20 @@ should be included in your classpath to show a reasonable level of log output:
     <logger name="com.github.dockerjava" level="WARN"/>
     <logger name="org.zeroturnaround.exec" level="WARN"/>
 </configuration>
+```
+
+## Using custom utility images
+
+Testcontainers uses additional docker images under some modes of execution: 
+
+* `richnorth/ambassador:latest`, which is a TCP proxy used to expose Docker Compose container ports outside of the compose network.
+* `richnorth/vncrecorder:latest`, which is a VNC->FLV recorder, used for capturing Webdriver test videos.
+
+> *N.B.:* both these images use the 'latest' tag, which could potentially affect repeatability of tests and compatibility with Testcontainers _if the image is ever changed_. This is a [known issue](https://github.com/testcontainers/testcontainers-java/issues/276) which will be addressed in the future. The current 'latest' version of these images will never be changed until they are replaced by a new image altogether.
+
+If it is necessary to override these image names (e.g. when using a private registry), you should create a file named `testcontainers.properties` and place it on the classpath with the following content:
+
+```properties
+ambassador.container.image=replacement image name here
+vncrecorder.container.image=replacement image name here
 ```