add docker image

docker/Dockerfile

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+FROM alpine:3.6
+
+COPY restic /usr/bin
+
+ENTRYPOINT ["/usr/bin/restic"]

docker/README.md

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+# Docker image
+
+## Build
+
+From the root of this repository run:
+
+```
+./docker/build.sh
+```
+
+image name will be `restic/restic:latest`
+
+## Run
+
+Set environment variable `RESTIC_REPOSITORY` and map volume to directories and
+files like:
+
+```
+docker run --rm -ti \
+    -v $HOME/.restic/passfile:/pass \
+    -v $HOME/importantdirectory:/data \
+    -e RESTIC_REPOSITORY=rest:https://user:pass@hostname/ \
+    restic/restic -p /pass backup /data
+```

docker/build.sh

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+#!/bin/sh
+
+set -e
+
+echo "Build binary using golang docker image"
+docker run --rm -ti \
+    -v `pwd`:/go/src/github.com/restic/restic \
+    -w /go/src/github.com/restic/restic golang:1.8.3-alpine go run build.go
+
+echo "Build docker image restic/restic:latest"
+docker build --rm -t restic/restic:latest -f docker/Dockerfile .