Target dependancy is output of a command - docker

Question

I've got a Makefile with a docker recipe which builds a docker image by doing make docker. The recipe looks like this:
# Build the docker file
docker: .setup ${GOBLDDIR}/docker-image
I wanted to set this up so that the docker file isn't rebuilt if everything is up-to-date, so I set up the ${GOBLDDIR}/docker-image dependancy. That dependancy is just a text file with the docker image ID. Here's that recipe, which actually does the docker build:
# DO THE DOCKER BUILD
${GOBLDDIR}/docker-image: ${GOBLDDIR} ${GOSRCFILES} go.mod Makefile Dockerfile
# Vendor our dependencies so we don't have to download them in the docker container
go mod vendor
# Wherever the config lives, we need it to be in the Docker build context
mkdir -p ./.docker-files
cp ${TFOUT} ./.docker-files/config.json
# The go app is actually compiled within this dockerfile
docker build . -t ${GOCMD} --build-arg AWS_PROFILE=${AWS_PROFILE} --build-arg SVC=${GOCMD} --build-arg VERSTR=${VERSTR}
docker images ${GOCMD} -q > ${GOBLDDIR}/docker-image
docker tag ${GOCMD}:latest ${GOCMD}:dev
docker tag ${GOCMD}:latest ${GOCMD}:${VERSTR}
# Clean up the crap we created just to build the Dockerfile
rm -rf vendor/
rm -rf ./.docker-files
Maybe this is an insane design - feels that way to me. There are certianly times when it doesn't work. I'm open to other ideas.
In particular, is there a way to make a dependency not be a file, but be the result of a command? For example, something like: docker inspect -f '{{ .Created }}' MY_IMAGE_NAME?

Answer 1

Your approach is generally a good approach. You can add checking not necessarily for the timestamp of creation time, but whether the image is actually the one that you previously built (someone else may have built a newer image with a different contents that may not reflect your repository anymore).
In general make decides whether to make a target or not by comparing timestamps of dependencies, so they are most commonly files. The list of dependencies may be manipulated however with some logic, which allows you to run arbitrary checks.
In your scenario you already store the image ID in a file. This may now be used to check whether the current image ID is the same that we built previously. We may compare output of the same command (extracted to a variable for DRY-ness) with the stored contents upon dependencies evaluation; if they do not match, we issue dependency of FORCE which is .PHONY and therefore always out of date, effectively triggering target remake:
$ cat Makefile
docker-id = docker image ls -q $(DOCKER_IMAGE)
docker-image: Dockerfile $(if $(findstring $(shell $(docker-id)),$(file <docker-image)),,FORCE)
docker build . -t $(DOCKER_IMAGE)
$(docker-id) > $#
.PHONY: FORCE
Output:
# Initial image build
$ make docker-image DOCKER_IMAGE=test
docker build . -t test
Sending build context to Docker daemon 7.168 kB
Step 1/2 : FROM alpine
---> 965ea09ff2eb
Step 2/2 : RUN echo 16517
---> Running in 6a9601225516
16517
---> 54b651cb8912
Removing intermediate container 6a9601225516
Successfully built 54b651cb8912
docker image ls -q test > docker-image
# No files changed, image exists
$ make docker-image DOCKER_IMAGE=test
make: 'docker-image' is up to date.
# Changing Dockerfile forces rebuild
$ touch Dockerfile
$ make docker-image DOCKER_IMAGE=test
docker build . -t test
Sending build context to Docker daemon 7.168 kB
Step 1/2 : FROM alpine
---> 965ea09ff2eb
Step 2/2 : RUN echo 16517
---> Using cache
---> 54b651cb8912
Successfully built 54b651cb8912
docker image ls -q test > docker-image
# Mismatched id forces rebuild
$ echo foobar > docker-image
$ make docker-image DOCKER_IMAGE=test
docker build . -t test
Sending build context to Docker daemon 7.168 kB
Step 1/2 : FROM alpine
---> 965ea09ff2eb
Step 2/2 : RUN echo 16517
---> Using cache
---> 54b651cb8912
Successfully built 54b651cb8912
docker image ls -q test > docker-image
# Missing image forces rebuild
$ docker image rm test
Untagged: test:latest
Deleted: sha256:54b651cb8912a5d2505f1f92c8ea4ee367cdca0d3f8e6f1ebc69d0cf646ca72c
Deleted: sha256:3749695bf207f2ca0829d8faf0ef538af1e18e8c39228347f48fd6d7c149f73a
$ make docker-image DOCKER_IMAGE=test
docker build . -t test
Sending build context to Docker daemon 7.168 kB
Step 1/2 : FROM alpine
---> 965ea09ff2eb
Step 2/2 : RUN echo 16517
---> Running in 70f8b09a92b3
16517
---> 8bdfdfff9db2
Removing intermediate container 70f8b09a92b3
Successfully built 8bdfdfff9db2
docker image ls -q test > docker-image