VOL-546 Add support for proxy servers in build

- Propagate proxy environment variables into docker builds
- Generate mvn settings.xml based on proxy env vars and allow custom xml
- Update to latest docker installation within go-lang docker image (to avoid apt-key proxy issue)
- Allow additional parameters with DOCKER_BUILD_EXTRA_ARGS, for example --no-cache, --pull
- Also fixes 'overriding recipe for target' for voltha target

Change-Id: Ifa545a0262ecfe434ad230ddab4f61935c7244e0
(cherry picked from commit e0c9acbbd529b38da0adacdb6701cb0e37553119)
diff --git a/docker/config/mvn_settings.xml b/docker/config/mvn_settings.xml
new file mode 100644
index 0000000..26f9926
--- /dev/null
+++ b/docker/config/mvn_settings.xml
@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+<settings xmlns="http://maven.apache.org/SETTINGS/1.0.0"
+          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+          xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.0.0 http://maven.apache.org/xsd/settings-1.0.0.xsd">
+  <!--PROXY-->
+  <!--EXTRA-->
+</settings>