Add -S option to tar for efficient sparse file handling

Fixes issue #3282
Return error if Dockerfile is empty
Allow mtu to be configured at daemon start
Only parse upto the mountpoint in mountinfo
Merge pull request #3294 from discordianfish/3293-better-error-dockerfile-empty

Return error if Dockerfile is empty
Merge pull request #3291 from dineshs-altiscale/3282-sparse-files

Add -S option to tar for efficient sparse file handling
Merge pull request #3300 from crosbymichael/fix-mountinfo-parsing

Only parse upto the mountpoint in mountinfo
Merge pull request #3289 from crosbymichael/add-mtu-option

Allow mtu to be configured at daemon start