update buil process for confd 0.12

Dockerfile

@@ -1,17 +1,22 @@
 FROM smebberson/alpine-base:latest
 
-ENV CONFD_VERSION=v0.12.0-alpha3
+ENV VERSION=v0.12.0-alpha3
 
-RUN apk add --no-cache go git gcc musl-dev && \
-    git clone https://github.com/kelseyhightower/confd.git /src/confd && \
-    cd /src/confd && \
-    git checkout -q --detach "$CONFD_VERSION" && \
-    cd /src/confd/src/github.com/kelseyhightower/confd && \
-    GOPATH=/src/confd/vendor:/src/confd go build -a -installsuffix cgo -ldflags '-extld ld -extldflags -static' -x . && \
-    mv ./confd /bin/ && \
-    chmod +x /bin/confd && \
-    apk del go git gcc musl-dev && \
-rm -rf /src
+RUN apk add --no-cache go gcc musl-dev openssl tar git
 
-ADD /root /
+RUN mkdir -p /src/confd
+RUN wget -qO- https://github.com/kelseyhightower/confd/archive/$VERSION.tar.gz | tar -xz -C /src/confd --strip 1
 
+WORKDIR /src/confd
+ENV GOPATH /src/confd/vendor:/src/confd
+RUN go get -d
+RUN go build -a -installsuffix cgo -ldflags '-extld ld -extldflags -static' -x .
+RUN mv ./confd /bin/
+RUN chmod +x /bin/confd
+
+RUN apk del go gcc musl-dev openssl tar git
+RUN rm -rf /src
+
+WORKDIR /
+
+ADD /rootfs /