Use pre-prepared Docker image in CI to speed things up

script/builder_image/Dockerfile

@@ -0,0 +1,6 @@
+FROM rust:buster
+RUN apt-get update && \
+	apt-get install --yes libsodium-dev awscli python-pip && \
+	    rm -rf /var/lib/apt/lists/*
+RUN rustup component add rustfmt
+RUN pip install s3cmd

script/builder_image/Makefile

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+DOCKER=lxpz/garage_builder_amd64
+
+docker:
+	docker build -t $(DOCKER):$(TAG) .
+	docker push $(DOCKER):$(TAG)
+	docker tag $(DOCKER):$(TAG) $(DOCKER):latest
+	docker push $(DOCKER):latest
+