example: sessions: scale out using redis

Former-commit-id: a811648e0bdf83c289656426f8ba67d785b7d5cc

_examples/sessions/database/redis/Dockerfile

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+FROM golang:latest AS builder
+RUN apt-get update
+WORKDIR /go/src/app
+ENV GO111MODULE=on \
+    CGO_ENABLED=0 \
+    GOOS=linux \
+    GOARCH=amd64
+# Caching go modules and build the binary.
+COPY go.mod .
+RUN go mod download
+COPY . .
+RUN go install
+
+FROM scratch
+COPY --from=builder /go/bin/app .
+ENTRYPOINT ["./app"]