dockerfile: use builder pattern to reduce image size

Docker image size goes from 625mb to 67mb since we exclude the build
environment and source. We also include tini and bind-tools which is standard
practice in our images now.

There is a risk we now act slightly worse due to not including the go build
toolchain. I will validate this and rollback this change if that is indeed the
case.

Author: keegancsmith

Dockerfile

@@ -1,6 +1,22 @@
-FROM golang:alpine
+FROM golang:alpine AS builder
 
-COPY . /go/src/github.com/sourcegraph/go-langserver
-RUN cd /go/src/github.com/sourcegraph/go-langserver && go install .
-RUN apk add --no-cache openssh git curl
+RUN apk add --no-cache ca-certificates
+
+ENV CGO_ENABLED=0 GO111MODULE=on
+WORKDIR /go/src/github.com/google/zoekt
+
+# Cache dependencies
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . ./
+RUN go install .
+
+FROM alpine AS go-langserver
+
+RUN apk add --no-cache openssh git curl ca-certificates bind-tools tini
+
+COPY --from=builder /go/bin/* /usr/local/bin/
+
+ENTRYPOINT ["/sbin/tini", "--"]
 CMD ["go-langserver"]