chore(docker): added tfenv so users can control the terraform version

Dockerfile

@@ -5,6 +5,18 @@ WORKDIR $DEST_DIR
 RUN apk update && apk add git openssh
 RUN go build -o /usr/bin/twrapper ./cmd/twrapper
 
+FROM golang:1.16.1-alpine AS tfenv
+ARG DEST_DIR="/go/src/tfenv"
+ARG TFENV_VERSION="v2.2.0"
+RUN apk update && apk add git openssh && \
+    git clone https://github.com/tfutils/tfenv.git ${DEST_DIR}
+WORKDIR $DEST_DIR
+RUN git checkout tags/${TFENV_VERSION} -b ${TFENV_VERSION}
+
 FROM alpine:3
 COPY --from=build /usr/bin/twrapper /bin/twrapper
+COPY --from=tfenv /go/src/tfenv /opt/tfenv
+RUN apk update && apk add bash curl && \
+    ln -s /opt/tfenv/bin/terraform /usr/bin && \
+    ln -s /opt/tfenv/bin/tfenv /usr/bin
 ENTRYPOINT [ "/bin/twrapper" ]