In my scenario, I am ADDing a bunch of libraries packaged as jars, copied over in a target directory during build time. I would like that instruction to be cached across builds (unless new libraries are added obviously, which doesn't happen very often).

When ADDing a URL in a Dockerfile, ctime, atime and mtime are not considered for the hash (see https://github.com/dotcloud/docker/pull/2809/files#diff-d8fbf83f530e600c64298c1b18b04d4aR411). It would be interesting to expose a flag controlling that clobberTimes for local files/directories, to be able to utilize caches in cases where the metadata of a file doesn't matter. I am thinking about adding a --ignore-times or --clober-times flag to the ADD instruction but I can't see any option in the current set of instructions, so it might be considered an anti-pattern. For the effect of that flag to be completely consistent across image builders, the times should also be reset to a default value (0?), so that the use of this option produces the exact same image no matter what the times on the filesystem of the host building the image are.

I am willing to contribute a patch if there is some interest, and some agreement on the exact naming/semantics of that flag.

Running Docker 0.8.1 on Ubuntu 14.04.