Fix ADD caching issue with . prefixed path

Docker-DCO-1.0-Signed-off-by: Guillaume J. Charmes <charmes.guillaume@gmail.com> (github: creack)
Make vfs error more explicit

Docker-DCO-1.0-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Fix issue with file caching + prevent wrong cache hit

Docker-DCO-1.0-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Makes sure the GPG key fingerprint is inlined in the doc everywhere.

Instead of `curl $url | apt-get add -` use the ubuntu keyserver and a full
fingerprint to retreive the key. This makes the distribution of the GPG key
more secure an less likely to change even if 3rd party gains access to the
packages repository.

Docker-DCO-1.0-Signed-off-by: Jonas Pfenniger <zimbatm@zimbatm.com> (github: zimbatm)
Fixes key installation for other target urls in hack/install.sh

Docker-DCO-1.0-Signed-off-by: Jonas Pfenniger <zimbatm@zimbatm.com> (github: zimbatm)
Merge pull request #3474 from creack/fix_add_cache_issue

Fix ADD caching issue with . prefixed path
Merge pull request #3361 from zimbatm/gpg-key-secure-best

Makes sure the GPG fingerprint is inlined in the doc everywhere.
Disable compression for build. More space usage but much faster upload

Docker-DCO-1.0-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Bump to v0.7.4

Docker-DCO-1.0-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #3495 from crosbymichael/bump_v0.7.4

Bump v0.7.4
Merge branch 'release' into merege_release_v0.7.4

Docker-DCO-1.0-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Change version to v0.7.4-dev

Docker-DCO-1.0-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #3498 from crosbymichael/merge_release_v0.7.4

Merge release v0.7.4
Merge pull request #3496 from creack/disable_build_compression

Disable compression for build. More space usage but much faster upload