I discovered a few days ago that when building images, a cache generated from docker build will not be used by docker-compose build or vice versa. This happens both on my Mac laptop (Docker Machine, Docker v1.10.3 installed through Homebrew) and on an Ubuntu 14.04 server (Docker v1.10.2 installed through apt-get).

It is simple to reproduce. The directory tiny-app in https://github.com/SamirTalwar/docker-build-weirdness shows the issue:

$ docker build --tag=tiny-app .
Sending build context to Docker daemon 4.096 kB
Step 1 : FROM ruby
 ---> 0f58cbcb8dce
Step 2 : WORKDIR /app
 ---> Running in be80cb8a6324
 ---> 2972973d66d4
Removing intermediate container be80cb8a6324
Step 3 : COPY script ./
 ---> 77bc9f2b16eb
Removing intermediate container dc9beae186af
Step 4 : CMD ./script
 ---> Running in ea5f91334ea4
 ---> d69db9b758e0
Removing intermediate container ea5f91334ea4
Successfully built d69db9b758e0

$ docker-compose build
Building tiny-app
Step 1 : FROM ruby
 ---> 0f58cbcb8dce
Step 2 : WORKDIR /app
 ---> Using cache
 ---> 2972973d66d4
Step 3 : COPY script ./
 ---> abb5045392d4
Removing intermediate container 1b1450dd66eb
Step 4 : CMD ./script
 ---> Running in a0c7889ca68a
 ---> 354c8fcf4876
Removing intermediate container a0c7889ca68a
Successfully built 354c8fcf4876

From the COPY operation onwards, it no longer uses the cache.

I don't know, but I am pretty sure, that this is because the tarballs sent to the Docker server are different. Also in that repo are the uploaded tarballs and hex dumps (hexdump -C), of the same from the CLI and Compose, captured through a fake HTTP server, server.py in the repo. The only real differences seem to be the inclusion of the owner user and group names in the latter, and a flag in the file mode header (grep for 0100644 vs. 0000644) which I cannot find the life of me find documentation on. I'm sure this boils down to how docker-py works, not Docker Compose, but I also can't be sure it'll be considered a bug here as opposed to the server.

I could, of course, be way off.

Whatever the reason, I'd love it if we could reconcile these so the cache is used. Cheers!