ADD hashing not consistent with docker build

[nickstenning]

As discussed with @bfirsh on IRC today.

It seems that the mechanism used by compose to tar up the build context results in a different hash for ADD directives than that computed by docker build. This isn't a showstopper, but it does mean that for long and complicated builds work is done twice when it doesn't strictly need to be.

I've put a reduced testcase up as a Gist.

Steps to reproduce

$ git clone https://gist.github.com/d679a4691d559d18aea8.git testcase
$ cd testcase
$ docker build .
$ docker-compose build web

Expected behaviour

The docker-compose build command uses the cached images generated by the preceding docker build command.

Actual behaviour

The docker-compose build command uses the cached images up until the ADD directive, which busts the cache and forces the remaining steps to be run again:

$ docker build .
Sending build context to Docker daemon 57.34 kB
Sending build context to Docker daemon
Step 0 : FROM busybox
 ---> 4986bf8c1536
Step 1 : RUN sleep 1
 ---> Running in c47334fb1fc3
 ---> 9f52371a6b61
Removing intermediate container c47334fb1fc3
Step 2 : ADD foo.txt ./
 ---> bca0b7a05b92
Removing intermediate container 74179c343ef4
Step 3 : RUN sleep 1
 ---> Running in 22219c266d47
 ---> 88757489d22f
Removing intermediate container 22219c266d47
Successfully built 88757489d22f
$ docker-compose build web
Building web...
Step 0 : FROM busybox
 ---> 4986bf8c1536
Step 1 : RUN sleep 1
 ---> Using cache
 ---> 9f52371a6b61
Step 2 : ADD foo.txt ./
 ---> 4a57b6d9cb05
Removing intermediate container 0c23f52f873f
Step 3 : RUN sleep 1
 ---> Running in 9acbacbf5f5b
 ---> c27815938ce2
Removing intermediate container 9acbacbf5f5b
Successfully built c27815938ce2

[dnephin]

I believe this is related to #883

[thaJeztah]

Looks to be the same yes, also a simple repro-case in that issue; #883 (comment)

[dnephin]

Closing as duplicate of #883

We fixed some bugs with the docker-py .dockerignore implementation which may have fixed this.