Don't loose additional metadata fields

Signed-off-by: Simon Ferquel <simon.ferquel@docker.com>

cli/command/context_test.go

@@ -0,0 +1,27 @@
+package command
+
+import (
+	"encoding/json"
+	"testing"
+
+	"gotest.tools/v3/assert"
+)
+
+func TestDockerContextMetadataKeepAdditionalFields(t *testing.T) {
+	c := DockerContext{
+		Description:       "test",
+		StackOrchestrator: OrchestratorSwarm,
+		AdditionalFields: map[string]interface{}{
+			"foo": "bar",
+		},
+	}
+	jsonBytes, err := json.Marshal(c)
+	assert.NilError(t, err)
+	assert.Equal(t, `{"Description":"test","StackOrchestrator":"swarm","foo":"bar"}`, string(jsonBytes))
+
+	var c2 DockerContext
+	assert.NilError(t, json.Unmarshal(jsonBytes, &c2))
+	assert.Equal(t, c2.AdditionalFields["foo"], "bar")
+	assert.Equal(t, c2.StackOrchestrator, OrchestratorSwarm)
+	assert.Equal(t, c2.Description, "test")
+}