docker: use multi-stage build

By leveraging docker's multi-stage builds, the size of the final mq2prom
docker image is reduced from 409MB to 21.2MB.

This also adds extra hardening to the image, because a lot less packages
and intermediate artifacts are included as part of it.

As part of this commit, the go version used to build the binary inside
the docker image has been bumped to 1.20.11.

Fixes: #11

Signed-off-by: Juan Vidal Allende <[email protected]>

Dockerfile

@@ -1,6 +1,13 @@
-FROM golang:1.20.10-alpine
+# Builder phase, includes golang toolchain
+FROM golang:1.20.11-alpine3.18 as builder
+COPY . /src
+WORKDIR /src
+RUN go build -o mq2prom ./cmd
+
+# Runtime phase, contains bare alpine plus the built binary and the config file
+# IMPORTANT: keep the alpine version on the builder and the runtime base images aligned
+FROM alpine:3.18
 RUN mkdir /mq2prom
-COPY . /mq2prom
 WORKDIR /mq2prom
-RUN go build -o mq2prom ./cmd
-CMD ./mq2prom
+COPY --from=builder /src/mq2prom /src/config.yaml .
+CMD ["./mq2prom"]