Docker: build.bash bad date format with non-english system

[AndreaFinazzi]

In docker/build.bash#L39, the date command
$(date +%Y_%b_%d_%H%M)
could result in badly formatted tags (and docker build error) with non-English time formats (Korean, in our case).

We fixed with $(date +%F_%H%M) which involves only numerical characters.

[nkoenig]

Thanks. I put this patch in PR #802 in 3d5e50e.