Use a docker container for full builds

docker/README.md

@@ -0,0 +1,29 @@
+Docker Build
+============
+
+Official builds are produced using a Docker image specified by the
+Dockerfile in this directory. The following commands exactly reproduce
+the official build process.
+
+Create an image called `syncthing/build` with the build environment.
+
+```
+./build.sh docker-init
+```
+
+> This is a Debian based image containing the latest stable version of
+> Go set up for cross compilation. The cross compilation uses the
+> dynamically linked standard libraries and SSE instructions for amd64
+> builds, but static linking and minimal instruction set for the 386 and
+> arm builds. The command should be run in the main repo directory, as a
+> user with permission to perform Docker operations.
+
+Build the full set of supported binaries.
+
+```
+./build.sh docker
+```
+
+> This uses a temporary container with the image from above and a volume
+> mapped to the directory containing the source. Tests are run and
+> binary packages created.