Git commit reference mechanism changed

- Git commit is passed as a build argument as explained in README.md
- `.git` directory is ignored by Docker for a quicker build and smaller context
- the build arg VCS_REF is used both to tag the image and the Go program

.dockerignore

@@ -9,3 +9,4 @@ coverage.txt
 Dockerfile
 LICENSE
 test.sh
+.git

Dockerfile

@@ -7,16 +7,17 @@ FROM ${BASE_IMAGE_BUILDER}:${GO_VERSION}-alpine${ALPINE_VERSION} AS builder
 ARG GOARCH=amd64
 ARG GOARM
 ARG VERSION=0.2.4
-RUN apk --update add -q --progress git
+ARG VCS_REF=
 WORKDIR /go/src/github.com/jesseduffield/lazydocker/
 COPY ./ .
 RUN CGO_ENABLED=0 GOOS=linux GOARCH=${GOARCH} GOARM=${GOARM} go build -a -installsuffix cgo -ldflags="-s -w \
-    -X main.commit=$(git rev-parse HEAD) \
+    -X main.commit=${VCS_REF} \
     -X main.version=${VERSION} \
     -X main.buildSource=Docker"
 
 FROM ${BASE_IMAGE}:${ALPINE_VERSION}
 ARG VERSION=0.2.4
+ARG VCS_REF=
 LABEL org.label-schema.schema-version="1.0.0-rc1" \
     maintainer="jessedduffield@gmail.com" \
     org.label-schema.vcs-ref=$VCS_REF \

README.md

@@ -90,6 +90,8 @@ go get github.com/jesseduffield/lazydocker
         https://github.com/jesseduffield/lazydocker.git
         ```
 
+    You can also add `--build-arg VCS_REF=$(git rev-parse HEAD)` so that the go build and the container contain the commit used.
+
     </p></details>
 1. Run it