Improve documentation of docker buildArgs (#5871)

docs/content/en/schemas/v2beta16.json

@@ -1111,7 +1111,7 @@
           "x-intellij-html-description": "arguments passed to the docker build.",
           "default": "{}",
           "examples": [
-            "{\"key1\": \"value1\", \"key2\": \"value2\"}"
+            "{\"key1\": \"value1\", \"key2\": \"{{ .ENV_VAR }}\"}"
           ]
         },
         "cacheFrom": {

pkg/skaffold/docker/build_args_test.go

@@ -245,6 +245,43 @@ func TestCreateBuildArgsFromArtifacts(t *testing.T) {
 	}
 }
 
+func TestBuildArgTemplating(t *testing.T) {
+	envs := map[string]string{
+		"MY_KEY":    "abc123",
+		"SO_SECRET": "do not say it",
+	}
+
+	args := map[string]*string{
+		"MY_KEY":    util.StringPtr("{{ .MY_KEY}}"),
+		"SO_SECRET": util.StringPtr("{{ .SO_SECRET}}"),
+	}
+
+	dockerFile := `
+	ARG MY_KEY
+	ARG SO_SECRET
+	ARG foo3
+	ARG SKAFFOLD_GO_GCFLAGS
+	FROM bar1`
+
+	testutil.Run(t, "test_templating_from_env_variables", func(t *testutil.T) {
+		t.SetEnvs(envs)
+		artifact := &latestV1.DockerArtifact{
+			DockerfilePath: "Dockerfile",
+			BuildArgs:      args,
+		}
+
+		tmpDir := t.NewTempDir()
+		tmpDir.Write("./Dockerfile", dockerFile)
+		workspace := tmpDir.Path(".")
+
+		filledMap, err := EvalBuildArgs(config.RunModes.Dev, workspace, artifact.DockerfilePath, artifact.BuildArgs, nil)
+
+		t.CheckNil(err)
+		t.CheckMatches(*filledMap["MY_KEY"], "abc123")
+		t.CheckMatches(*filledMap["SO_SECRET"], "do not say it")
+	})
+}
+
 type mockArtifactResolver struct {
 	m map[string]string
 }

pkg/skaffold/schema/latest/v1/config.go

@@ -1205,7 +1205,7 @@ type DockerArtifact struct {
 	Target string `yaml:"target,omitempty"`
 
 	// BuildArgs are arguments passed to the docker build.
-	// For example: `{"key1": "value1", "key2": "value2"}`.
+	// For example: `{"key1": "value1", "key2": "{{ .ENV_VAR }}"}`.
 	BuildArgs map[string]*string `yaml:"buildArgs,omitempty"`
 
 	// NetworkMode is passed through to docker and overrides the