Adds a Dockerfile that creates a terragrunt image

Dockerfile

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+FROM golang:1.16.3-alpine as builder
+
+RUN apk add --update --no-cache make git
+
+WORKDIR /go/src/terragrunt
+
+# Download modules in a separate layer so docker caches it, to reduce build time
+# when modules have not changed.
+COPY go.mod .
+COPY go.sum .
+RUN go mod download -x
+
+COPY . .
+
+RUN make build
+
+###
+
+FROM alpine:latest
+
+COPY --from=builder /go/src/terragrunt/terragrunt /usr/local/bin/
+
+ENTRYPOINT terragrunt