builder: strip blank lines before processing any line continuations.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
builder: comments should also be elided in the middle of statements following a line continuation.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #8580 from erikh/filter_comments_first

builder: strip blank lines before processing any line continuations.