docker: Default settings should provide a volume for git-cache

[jnohlgard]

Description

Running the build in a Docker container with the BUILD_IN_DOCKER=1 make command should provide a persistent volume for storing the git-cache directory to reduce network download traffic.

Steps to reproduce the issue

in examples/ccn-lite-relay:

BUILD_IN_DOCKER=1 make

Expected results

git sources should be loaded from the git-cache

Actual results

git sources are re-downloaded whenever the bin directory is deleted

Versions

riot/riotbuild docker image b329486bb877 from 2017-12-01T14:36:49.484Z
up-to-date master branch from 2017-12-14

[cladmi]

I will look at it.
I think it would require to move the default out of dist/tools/git/git-cache:

GIT_CACHE_DIR=${GIT_CACHE_DIR:-${HOME}/.gitcache}

To have access to the value in the build system.

[cladmi]

I proposed something in #9651

The difference from this issue description is that you say "default settings" whereas I only did it if the ${GIT_CACHE_DIR} is defined and exists, I do not create one.