docker: provide test image with source

Makefile

@@ -182,6 +182,9 @@ binaries: prepare swagger  ## Build binary releases (FreeBSD, macOS, Linux gener
 docker-image:  ## Build aptly-dev docker image
 	@docker build -f system/Dockerfile . -t aptly-dev
 
+docker-image-test:  # Build aptly-test docker image for testing
+	@docker build -f docker/test.Dockerfile . -t aptly-test
+
 docker-image-no-cache:  ## Build aptly-dev docker image (no cache)
 	@docker build --no-cache -f system/Dockerfile . -t aptly-dev
 

docker/test.Dockerfile

@@ -0,0 +1,7 @@
+FROM aptly-dev
+
+ADD --chown=aptly:aptly . /work/src/
+
+# Pre-populate the Go module cache so go mod verify works offline
+RUN chown aptly /work/src && mkdir -p /work/src/.go && chown aptly /work/src/.go && \
+    cd /work/src && sudo -u aptly GOPATH=/work/src/.go GOCACHE=/work/src/.go/cache go mod download