Add support for compressing build context during image build

When sending a build context to a remote server it may be
(significantly) advantageous to compress the build context. This commit
adds support for gz compression when constructing a build context
using a command like "docker build --compress ."

Signed-off-by: Paul Kehrer <paul.l.kehrer@gmail.com>

man/docker-build.1.md

@@ -16,6 +16,7 @@ docker-build - Build a new image from the source code at PATH
 [**--label**[=*[]*]]
 [**--no-cache**]
 [**--pull**]
+[**--compress**]
 [**-q**|**--quiet**]
 [**--rm**[=*true*]]
 [**-t**|**--tag**[=*[]*]]
@@ -84,6 +85,9 @@ set as the **URL**, the repository is cloned locally and then sent as the contex
 **--pull**=*true*|*false*
    Always attempt to pull a newer version of the image. The default is *false*.
 
+**--compress**=*true*|*false*
+    Compress the build context using gzip. The default is *false*.
+
 **-q**, **--quiet**=*true*|*false*
    Suppress the build output and print image ID on success. The default is *false*.