added docker image support

* added environment variable support
* added docker instructions
* added .env hint

Dockerfile

@@ -0,0 +1,27 @@
+FROM golang:1.21-alpine as builder
+
+RUN apk update
+RUN apk upgrade
+RUN apk add --no-cache git make
+
+RUN git --version
+
+WORKDIR /work
+
+COPY go.mod .
+COPY . .
+RUN go mod download
+RUN make
+
+FROM alpine:latest
+LABEL maintainer="Thomas von Dein <git@daemon.de>"
+
+WORKDIR /app
+COPY --from=builder /work/kleingebaeck /app/kleingebaeck
+
+ENV KLEINGEBAECK_OUTDIR /backup
+ENV LANG C.UTF-8
+USER 1001:1001
+
+ENTRYPOINT ["/app/kleingebaeck"]
+CMD ["-h"]