fix: default Dockerfile build to the runnable agent stage (#246)

When called without a target, docker build produces an image based on
the final stage in a multi-stage Dockerfile. This rearranges our
Dockerfile so that the default is the runnable agent image and not the
final state of compiling and testing.

Co-authored-by: Jamie Danielson <jamieedanielson@gmail.com>
Co-authored-by: Mike Goldsmith <goldsmith.mike@gmail.com>

Dockerfile

@@ -1,3 +1,5 @@
+# uses docker multi-stage builds: https://docs.docker.com/build/building/multi-stage/
+# base stage builds the agent binary for use in later stages
 FROM golang:1.21 as base
 RUN apt update -yq && apt install -yq make libpcap-dev
 WORKDIR /src
@@ -6,10 +8,13 @@ RUN go mod download
 COPY . .
 RUN make build
 
-FROM ubuntu:22.04 as build
+# run tests with 'docker build --target test .'; skips the runnable image build
+FROM base as test
+RUN make test
+
+# last unnamed stage is the default target for any image build
+# this produces the runnable agent image
+FROM ubuntu:22.04
 RUN apt-get update -yq && apt-get install -yq ca-certificates libpcap-dev
 COPY --from=base /src/hny-network-agent /bin/hny-network-agent
 ENTRYPOINT [ "/bin/hny-network-agent" ]
-
-FROM base as test
-RUN make test

Makefile

@@ -8,7 +8,7 @@ build:
 .PHONY: docker-build
 #: build the agent image
 docker-build:
-	docker build --target build --tag $(IMG_NAME):$(IMG_TAG) .
+	docker build --tag $(IMG_NAME):$(IMG_TAG) .
 
 .PHONY: test
 #: run unit tests