Docker: build rest-server at container build time

Using docker's multi-stage builds we can build the restic/rest-server
within a golang build environment then create a container for use
(without the build environment) in a second build stage.

The advantages are:

1. Building the rest-server is predictable in a pristine environment
   each time.
2. Container builds ensure we get the latest rest-server every time.

Updated README with details of new docker build approach, and added
changelog for unreleased changes.

Dockerfile

@@ -1,3 +1,14 @@
+FROM golang:alpine AS builder
+
+ENV CGO_ENABLED 0
+
+COPY . /build
+WORKDIR /build
+RUN go build -o rest-server ./cmd/rest-server
+
+
+
+
 FROM alpine
 
 ENV DATA_DIRECTORY /data
@@ -8,7 +19,7 @@ RUN apk add --no-cache --update apache2-utils
 COPY docker/create_user /usr/bin/
 COPY docker/delete_user /usr/bin/
 COPY docker/entrypoint.sh /entrypoint.sh
-COPY rest-server /usr/bin
+COPY --from=builder /build/rest-server /usr/bin
 
 VOLUME /data
 EXPOSE 8000