add Dockerfile for smtprelay

Dockerfile

@@ -0,0 +1,22 @@
+# Package versions
+ARG GOLANG_VERSION="1.25.4"
+
+# Stage 1: Build
+FROM core.harbor.brds.ca/library/golang:${GOLANG_VERSION} AS builder
+WORKDIR /project
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+RUN go build .
+
+# Stage 2: Package
+FROM scratch
+LABEL org.opencontainers.image.title="smtprelay"
+LABEL org.opencontainers.image.authors="drew@brds.ca"
+LABEL org.opencontainers.image.description="Image containing the smtprelay program."
+
+COPY --from=builder /project/smtprelay /
+
+# Command to run
+ENTRYPOINT ["/smtprelay"]