build: 🏗️ add Dockerfile

Dockerfile

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+FROM golang:1.21
+
+# Set destination for COPY
+WORKDIR /app
+
+# Download Go modules
+COPY go.mod go.sum ./
+RUN go mod download
+
+# Copy the source code. Note the slash at the end, as explained in
+# https://docs.docker.com/engine/reference/builder/#copy
+COPY *.go ./
+
+# Build
+RUN CGO_ENABLED=0 GOOS=linux go build -o /listy
+
+# Optional:
+# To bind to a TCP port, runtime parameters must be supplied to the docker command.
+# But we can document in the Dockerfile what ports
+# the application is going to listen on by default.
+# https://docs.docker.com/engine/reference/builder/#expose
+EXPOSE 5001
+
+# Run
+CMD ["/listy"]