docker: Update Dockerfile build instructions

Signed-off-by: aeneasr <aeneas@ory.sh>

Dockerfile

@@ -1,27 +1,14 @@
-FROM golang:1.11-alpine
+# To compile this image manually run:
+#
+# $ GO111MODULE=on GOOS=linux GOARCH=amd64 go build && docker build -t oryd/oathkeeper . && rm oathkeeper
+FROM alpine:3.9
 
-ARG git_tag
-ARG git_commit
-
-RUN apk add --no-cache git build-base
-
-WORKDIR /go/src/github.com/ory/oathkeeper
-
-ENV GO111MODULE=on
-
-ADD ./go.mod ./go.mod
-ADD ./go.sum ./go.sum
-
-RUN go mod download
-
-ADD . .
-
-RUN go mod verify
-RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -ldflags "-s -X github.com/ory/oathkeeper/cmd.Version=$git_tag -X github.com/ory/oathkeeper/cmd.BuildTime=`TZ=UTC date -u '+%Y-%m-%dT%H:%M:%SZ'` -X github.com/ory/oathkeeper/cmd.GitHash=$git_commit" -a -installsuffix cgo -o oathkeeper
+RUN apk add -U --no-cache ca-certificates
 
 FROM scratch
 
 COPY --from=0 /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
-COPY --from=0 /go/src/github.com/ory/oathkeeper/oathkeeper /usr/bin/oathkeeper
+COPY oathkeeper /usr/bin/oathkeeper
+COPY .releaser/LICENSE.txt /LICENSE.txt
 
 ENTRYPOINT ["oathkeeper"]