diff --git a/lib/ex_docker_build/docker_build.ex b/lib/ex_docker_build/docker_build.ex
index 1e25a55..c794930 100644
--- a/lib/ex_docker_build/docker_build.ex
+++ b/lib/ex_docker_build/docker_build.ex
@@ -160,8 +160,12 @@ defmodule ExDockerBuild.DockerBuild do
     |> ExDockerBuild.create_layer()
   end
 
-  # defp exec({"ARG", args}, context, _path) do
-  # end
+  defp exec({"ARG", args}, context, _path) do
+    args = String.split(args, "=")
+
+    Map.merge(context, %{"Args" => Enum.at(args, 2)})
+    |> ExDockerBuild.create_layer()
+  end
 
   # Supports:
   # VOLUME volume_name for named volumes
diff --git a/mix.lock b/mix.lock
index 3e8cb77..e6d95a9 100644
--- a/mix.lock
+++ b/mix.lock
@@ -22,6 +22,6 @@
   "redbug": {:hex, :redbug, "1.2.1", "9153ee50e42c39ce3f6efa65ee746f4a52896da66862cfb59e7c0f838b7b8414", [:rebar3], [], "hexpm"},
   "rexbug": {:hex, :rexbug, "1.0.0", "52b5d25c423f1c31ecce6960a556cac6391f62d61945f305248b07149567456c", [:mix], [{:redbug, "~> 1.2", [hex: :redbug, repo: "hexpm", optional: false]}], "hexpm"},
   "ssl_verify_fun": {:hex, :ssl_verify_fun, "1.1.1", "28a4d65b7f59893bc2c7de786dec1e1555bd742d336043fe644ae956c3497fbe", [:make, :rebar], [], "hexpm"},
-  "tracer": {:git, "git@github.com:gabiz/tracer.git", "4659bf3cc9e0cc86e653794b800e9eb5bbe9f0a2", [branch: "master"]},
+  "tracer": {:git, "https://github.com/gabiz/tracer.git", "4659bf3cc9e0cc86e653794b800e9eb5bbe9f0a2", [branch: "master"]},
   "unicode_util_compat": {:hex, :unicode_util_compat, "0.3.1", "a1f612a7b512638634a603c8f401892afbf99b8ce93a45041f8aaca99cadb85e", [:rebar3], [], "hexpm"},
 }
diff --git a/test/integration/docker_build_test.exs b/test/integration/docker_build_test.exs
index 4a46522..883b96c 100644
--- a/test/integration/docker_build_test.exs
+++ b/test/integration/docker_build_test.exs
@@ -259,4 +259,50 @@ defmodule ExDockerBuild.Integration.DockerBuildTest do
       assert "/bin/sh -c #(nop) ADD file:" <> <<_::binary-size(64)>> <> " in / " = command
     end
   end
+
+  describe "defining variables at build-time" do
+    test "setting ARG instruction" do
+      instructions = [
+        {"FROM", "alpine:latest"},
+        {"ARG", "tag=\"latest\""},
+        {"RUN", "mkdir /myvol"},
+        {"RUN", "echo \"hello-world!!!!\" > /myvol/greeting"}
+      ]
+
+      log =
+        capture_log(fn ->
+          assert {:ok, image_id} = DockerBuild.build(instructions, "")
+        end)
+
+      assert log =~ "STEP 1/4 : FROM alpine:latest"
+      assert log =~ "pulling image alpine:latest"
+      assert log =~ "STEP 2/4 : ARG tag=\"latest\""
+      assert log =~ "STEP 3/4 : RUN mkdir /myvol"
+      assert log =~ "STEP 4/4 : RUN echo \"hello-world!!!!\" > /myvol/greeting"
+    end
+
+    test "setting multiple args" do
+      instructions = [
+        {"FROM", "alpine:latest"},
+        {"ARG", "tag=\"latest\""},
+        {"ARG", "A_VARIABLE="},
+        {"ENV", "an_env_var=$A_VARIABLE"},
+        {"RUN", "mkdir /myvol"},
+        {"RUN", "echo \"hello-world!!!!\" > /myvol/greeting"}
+      ]
+
+      log =
+        capture_log(fn ->
+          assert {:ok, image_id} = DockerBuild.build(instructions, "")
+        end)
+
+      assert log =~ "STEP 1/6 : FROM alpine:latest"
+      assert log =~ "pulling image alpine:latest"
+      assert log =~ "STEP 2/6 : ARG tag=\"latest\""
+      assert log =~ "STEP 3/6 : ARG A_VARIABLE="
+      assert log =~ "STEP 4/6 : ENV an_env_var=$A_VARIABLE"
+      assert log =~ "STEP 5/6 : RUN mkdir /myvol"
+      assert log =~ "STEP 6/6 : RUN echo \"hello-world!!!!\" > /myvol/greeting"
+    end
+  end
 end