Add dockerEnv target to customize environment passed to docker command

contrib/docker/src/mill/contrib/docker/DockerModule.scala

@@ -65,6 +65,16 @@ trait DockerModule { outer: JavaModule =>
      */
     def envVars: T[Map[String, String]] = Map.empty[String, String]
 
+    /**
+     * Environment to pass to the docker commands.
+     * Example: DOCKER_DEFAULT_PLATFORM=linux/amd64
+     *
+     * See also the Docker docs on
+     * [[https://docs.docker.com/engine/reference/commandline/cli/#environment-variables Environment variables]]
+     * for more information.
+     */
+    def dockerEnv: T[Map[String, String]] = T.env
+
     /**
      * Commands to add as RUN instructions.
      *
@@ -138,19 +148,21 @@ trait DockerModule { outer: JavaModule =>
     }
 
     private def pullAndHash = Task.Input {
+      val env = dockerEnv()
       def imageHash() =
         os.proc(executable(), "images", "--no-trunc", "--quiet", baseImage())
-          .call(stderr = os.Inherit).out.text().trim
+          .call(stderr = os.Inherit, env = env).out.text().trim
 
       if (pullBaseImage() || imageHash().isEmpty)
         os.proc(executable(), "image", "pull", baseImage())
-          .call(stdout = os.Inherit, stderr = os.Inherit)
+          .call(stdout = os.Inherit, stderr = os.Inherit, env = env)
 
       (pullBaseImage(), imageHash())
     }
 
     final def build = Task {
       val dest = T.dest
+      val env = dockerEnv()
 
       val asmPath = outer.assembly().path
       os.copy(asmPath, dest / asmPath.last)
@@ -168,7 +180,7 @@ trait DockerModule { outer: JavaModule =>
         if (platform().nonEmpty)
           log.info("Platform parameter is ignored when using non-docker executable")
         os.proc(executable(), "build", tagArgs, pullLatestBase, dest)
-          .call(stdout = os.Inherit, stderr = os.Inherit)
+          .call(stdout = os.Inherit, stderr = os.Inherit, env = env)
       } else {
         os.proc(
           executable(),
@@ -180,7 +192,7 @@ trait DockerModule { outer: JavaModule =>
           platform(),
           dest
         )
-          .call(stdout = os.Inherit, stderr = os.Inherit)
+          .call(stdout = os.Inherit, stderr = os.Inherit, env = env)
       }
       log.info(s"Docker build completed ${if (result.exitCode == 0) "successfully"
         else "unsuccessfully"} with ${result.exitCode}")
@@ -189,8 +201,9 @@ trait DockerModule { outer: JavaModule =>
 
     final def push() = Task.Command {
       val tags = build()
+      val env = dockerEnv()
       tags.foreach(t =>
-        os.proc(executable(), "push", t).call(stdout = os.Inherit, stderr = os.Inherit)
+        os.proc(executable(), "push", t).call(stdout = os.Inherit, stderr = os.Inherit, env = env)
       )
     }
   }

contrib/docker/test/src/mill/contrib/docker/DockerModuleTest.scala

@@ -2,6 +2,8 @@ package mill
 package contrib.docker
 
 import mill.scalalib.JavaModule
+import mill.api.Result
+import mill.util.TestUtil
 import mill.testkit.UnitTester
 import mill.testkit.TestBaseModule
 import os.Path
@@ -41,6 +43,10 @@ object DockerModuleTest extends TestSuite {
       override def executable = testExecutable
       override def jvmOptions = Seq("-Xmx1024M")
     }
+
+    object dockerEnv extends DockerConfig {
+      override def dockerEnv = Map("DOCKER_HOST" -> "wrong_host")
+    }
   }
 
   val testArtifactName = "mill-docker-contrib-test"
@@ -86,6 +92,15 @@ object DockerModuleTest extends TestSuite {
         val Right(result) = eval(Docker.dockerAll.build)
         assert(result.value == List(testArtifactName))
       }
+
+      "dockerEnv" - workspaceTest(Docker) { eval =>
+        // since stdout and stderr are inherited we can only test
+        // that docker fails with wrong DOCKER_HOST
+        val Left(Result.Exception(error: os.SubprocessException, _)) =
+          eval(Docker.dockerEnv.build)
+        val message = error.getMessage
+        assert(message == "Result of docker…: 1\n")
+      }
     }
 
     test("dockerfile contents") {