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

README.md

@@ -19,11 +19,15 @@ The required version of restic backup client to use with `rest-server` is [v0.7.
 
 For building the `rest-server` binary run `CGO_ENABLED=0 go build -o rest-server ./cmd/rest-server`
 
+
 ## Docker
 
 ### Build image
 
-Put the `rest-server` binary in the current directory, then run:
+The docker image build process will build a fresh version of the rest-server and package that into
+a usable container.
+
+To build the binary along with the container run:
 
     docker build -t restic/rest-server:latest .
 
@@ -32,6 +36,7 @@ Put the `rest-server` binary in the current directory, then run:
 
     docker pull restic/rest-server
 
+
 ## Usage
 
 To learn how to use restic backup client with REST backend, please consult [restic manual](https://restic.readthedocs.io/en/latest/030_preparing_a_new_repo.html#rest-server).

changelog/unreleased/issue-146

@@ -0,0 +1,5 @@
+Build rest-server at docker container build time
+
+Add a build stage such that the latest rest-server is always built and packaged.
+This is done in a standard golang container to ensure a clean build environment
+and only the final binary is shipped rather than the whole build environment.