Add docker configs

Dockerfile

+FROM golang:alpine as builder
+
+RUN apk add --no-cache git
+
+WORKDIR /src
+COPY go.* ./
+RUN go mod download
+COPY *.go ./
+RUN CGO_ENABLED=0 GOOS=linux go build .
+
+FROM scratch
+COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
+COPY --from=builder /src/bahn-proxy /bahn-proxy
+COPY assets /assets
+ENTRYPOINT ["/bahn-proxy"]

build.sh

+#!/bin/sh
+IMAGE=k8r.eu/justjanne/bahn-proxy
+TAGS=$(git describe --always --tags HEAD)
+
+docker build -t $IMAGE:$TAGS .
+docker tag $IMAGE:$TAGS $IMAGE:latest
+echo Successfully tagged $IMAGE:latest
+docker push $IMAGE:$TAGS
+docker push $IMAGE:latest
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