Improve support for alternative container runtimes

In several cases, alternative container runtimes offers a docker
compatible API and populates the docker context accordingly.

However, in the current implementation of testcontainers, the context is
often ignored as the `EnvironmentAndSystemPropertyClientProviderStrategy` only considers
the `DOCKER_HOST` override either through testcontainers property or the
environment variables.

Improve the support of multiple container runtimes by honoring the
current docker context.

In addition, improve the detection of whether the Docker engine runs
in a virtual machine without root access for the current user, so it
removes the need to configure `TESTCONTAINERS_DOCKER_SOCKET_OVERRIDE` in
standard cases.

This change has been tested with:
- colima
- rancher desktop (in non-privileged mode)
- docker desktop
- orbstack

Author: tjamet

core/src/main/java/org/testcontainers/DockerClientFactory.java

@@ -30,8 +30,11 @@
 import org.testcontainers.utility.ResourceReaper;
 import org.testcontainers.utility.TestcontainersConfiguration;
 
+import java.nio.file.Files;
+import java.io.IOException;
 import java.io.InputStream;
 import java.net.URI;
+import java.nio.file.Paths;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
@@ -174,10 +177,25 @@ public String getRemoteDockerUnixSocketPath() {
         }
 
         URI dockerHost = getTransportConfig().getDockerHost();
-        String path = "unix".equals(dockerHost.getScheme()) ? dockerHost.getRawPath() : "/var/run/docker.sock";
+
+        String path = this.isRemoteDockerDaemon(dockerHost) ?  "/var/run/docker.sock" : dockerHost.getRawPath();
         return SystemUtils.IS_OS_WINDOWS ? "/" + path : path;
     }
 
+    private Boolean isRemoteDockerDaemon(URI dockerHost){
+        if (! "unix".equals(dockerHost.getScheme())) {
+            return true;
+        }
+        // Several privileged less Docker providers exposes the Docker socket within the user's home directory, owned by the users.
+        // This is the case for Rancher Desktop for example.
+        // In this case, it is likely that the Docker daemon running in the virtual machine listens on /var/run/docker.sock
+        try {
+            return Files.getOwner(Paths.get(dockerHost.getRawPath())).getName().equals(System.getProperty("user.name"));
+        } catch (IOException e) {
+            return false;
+        }
+    }
+
     /**
      * @return a new initialized Docker client
      */

core/src/main/java/org/testcontainers/dockerclient/EnvironmentAndSystemPropertyClientProviderStrategy.java

@@ -9,14 +9,28 @@
 
 /**
  * Use environment variables and system properties (as supported by the underlying DockerClient DefaultConfigBuilder)
- * to try and locate a docker environment.
+ * to try and locate a docker environment based on the currently active configuration.
  * <p>
- * Resolution order is:
+ * Docker Host resolution order is:
  * <ol>
  *     <li>DOCKER_HOST env var</li>
  *     <li>docker.host in ~/.testcontainers.properties</li>
+ *     <li>docker host pointed by the Docker context</li>
  * </ol>
- *
+ * <p>
+ * Docker context resolution order is
+ * <ol>
+ *     <li>DOCKER_CONTEXT env var</li>
+ *     <li>docker.context in ~/.testcontainers.properties</li>
+ *     <li>current docker context pointed by the Docker config</li>
+ * </ol>
+ * <p>
+ * Docker config resolution order is
+ * <ol>
+ *     <li>DOCKER_CONFIG env var</li>
+ *     <li>$HOME/.docker</li>
+ * </ol>
+ * 
  * @deprecated this class is used by the SPI and should not be used directly
  */
 @Deprecated
@@ -43,9 +57,14 @@ public EnvironmentAndSystemPropertyClientProviderStrategy() {
             case "auto":
                 Optional<String> dockerHost = getSetting("docker.host");
                 dockerHost.ifPresent(configBuilder::withDockerHost);
-                applicable = dockerHost.isPresent();
+                getSetting("docker.config").ifPresent(configBuilder::withDockerConfig);
+                getSetting("docker.context").ifPresent(configBuilder::withDockerContext);
                 getSetting("docker.tls.verify").ifPresent(configBuilder::withDockerTlsVerify);
                 getSetting("docker.cert.path").ifPresent(configBuilder::withDockerCertPath);
+                // We don't have access to the docker-java internals to understand whether a docker-context is defined or not.
+                // Therefore, we assume there is always a docker context defined and try it to start with.
+                // In case there is an error initializing the docker client with this library, it will fallback to the other resolvers.
+                applicable = true;
                 break;
             case "autoIgnoringUserProperties":
                 applicable = configBuilder.isDockerHostSetExplicitly();

docs/supported_docker_environment/index.md

@@ -11,7 +11,7 @@ These Docker environments are automatically detected and used by Testcontainers
 
 It is possible to configure Testcontainers to work with alternative container runtimes.
 Making use of the free [Testcontainers Desktop](https://testcontainers.com/desktop/) app will take care of most of the manual configuration.
-When using those alternatives without Testcontainers Desktop, 
+Although Testcontainers has a detection for most of the cases, when using those alternatives without Testcontainers Desktop, 
 sometimes some manual configuration might be necessary 
 (see further down for specific runtimes, or [Customizing Docker host detection](/features/configuration/#customizing-docker-host-detection) for general configuration mechanisms).
 Alternative container runtimes are not actively tested in the main development workflow,
@@ -21,7 +21,14 @@ please contact the Testcontainers team and other users from the Testcontainers c
 
 ## Colima
 
-In order to run testcontainers against [colima](https://github.com/abiosoft/colima) the env vars below should be set
+In order to run testcontainers against [colima](https://github.com/abiosoft/colima) the standard configuration should be automatically detected. Starting colima should be enough.
+
+```bash
+colima start --network-address
+docker context use colima
+```
+
+For more complex scenarios you should configure the following environment variables:
 
 ```bash
 colima start --network-address
@@ -61,25 +68,9 @@ export TESTCONTAINERS_RYUK_DISABLED=true
 
 ## Rancher Desktop
 
-In order to run testcontainers against [Rancher Desktop](https://rancherdesktop.io/) the env vars below should be set.
-
-If you're running Rancher Desktop as an administrator in a MacOS (M1) machine:
-
-Using QEMU emulation
-
-```bash
-export TESTCONTAINERS_HOST_OVERRIDE=$(rdctl shell ip a show rd0 | awk '/inet / {sub("/.*",""); print $2}')
-```
-
-Using VZ emulation
-
-```bash
-export TESTCONTAINERS_HOST_OVERRIDE=$(rdctl shell ip a show vznat | awk '/inet / {sub("/.*",""); print $2}')
-```
-
-If you're not running Rancher Desktop as an administrator in a MacOS (M1) machine:
+In order to run testcontainers against [Rancher Desktop](https://rancherdesktop.io/) the standard configuration should be automatically detected. Starting colima should be enough.
 
-Using VZ emulation
+For more complex scenarios you should configure the following environment variables:
 
 ```bash
 export DOCKER_HOST=unix://$HOME/.rd/docker.sock
@@ -95,10 +86,15 @@ Testcontainers will try to connect to a Docker daemon using the following strate
 	* `DOCKER_HOST`
 	* `DOCKER_TLS_VERIFY`
 	* `DOCKER_CERT_PATH`
+	* `DOCKER_CONTEXT`
+	* `DOCKER_CONFIG`
 * Defaults:
 	* `DOCKER_HOST=https://localhost:2376`
 	* `DOCKER_TLS_VERIFY=1`
 	* `DOCKER_CERT_PATH=~/.docker`
+	* `DOCKER_CONTEXT=$(docker context show)`
+	* `DOCKER_CONFIG=~/.docker`
+* If the current docker context provides a working Docker environment, it will be preferred to further installation detection.
 * If Docker Machine is installed, the docker machine environment for the *first* machine found. Docker Machine needs to be on the PATH for this to succeed.
 * If you're going to run your tests inside a container, please read [Patterns for running tests inside a docker container](continuous_integration/dind_patterns.md) first.