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/image.less

@@ -0,0 +1,33 @@
+// Image Mixins
+// - Responsive image
+// - Retina image
+
+
+// Responsive image
+//
+// Keep images from scaling beyond the width of their parents.
+.img-responsive(@display: block) {
+  display: @display;
+  max-width: 100%; // Part 1: Set a maximum relative to the parent
+  height: auto; // Part 2: Scale the height according to the width, otherwise you get stretching
+}
+
+
+// Retina image
+//
+// Short retina mixin for setting background-image and -size. Note that the
+// spelling of `min--moz-device-pixel-ratio` is intentional.
+.img-retina(@file-1x; @file-2x; @width-1x; @height-1x) {
+  background-image: url("@{file-1x}");
+
+  @media
+  only screen and (-webkit-min-device-pixel-ratio: 2),
+  only screen and (   min--moz-device-pixel-ratio: 2),
+  only screen and (     -o-min-device-pixel-ratio: 2/1),
+  only screen and (        min-device-pixel-ratio: 2),
+  only screen and (                min-resolution: 192dpi),
+  only screen and (                min-resolution: 2dppx) {
+    background-image: url("@{file-2x}");
+    background-size: @width-1x @height-1x;
+  }
+}