Build modified streamline images

The official image from streamline does not currently work for the arm64
platform. As a temporary measure, the source code and docker build scripts
have been lifted from the official images and are used to build locally.

Some additional modifications are made to reduce overall image size, these
are documented in docker/README.md

docker/streamline-src/public/elite/bootstrap/less/mixins/clearfix.less

@@ -0,0 +1,22 @@
+// Clearfix
+//
+// For modern browsers
+// 1. The space content is one way to avoid an Opera bug when the
+//    contenteditable attribute is included anywhere else in the document.
+//    Otherwise it causes space to appear at the top and bottom of elements
+//    that are clearfixed.
+// 2. The use of `table` rather than `block` is only necessary if using
+//    `:before` to contain the top-margins of child elements.
+//
+// Source: http://nicolasgallagher.com/micro-clearfix-hack/
+
+.clearfix() {
+  &:before,
+  &:after {
+    content: " "; // 1
+    display: table; // 2
+  }
+  &:after {
+    clear: both;
+  }
+}