docs: add missing cache-from man docs

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

man/docker-build.1.md

@@ -8,6 +8,7 @@ docker-build - Build an image from a Dockerfile
 **docker build**
 [**--add-host**[=*[]*]]
 [**--build-arg**[=*[]*]]
+[**--cache-from**[=*[]*]]
 [**--cpu-shares**[=*0*]]
 [**--cgroup-parent**[=*CGROUP-PARENT*]]
 [**--help**]
@@ -94,6 +95,9 @@ option can be set multiple times.
    or for variable expansion in other Dockerfile instructions. This is not meant
    for passing secret values. [Read more about the buildargs instruction](https://docs.docker.com/engine/reference/builder/#arg)
 
+**--cache-from**=""
+   Set image that will be used as a build cache source.
+
 **--force-rm**=*true*|*false*
    Always remove intermediate containers, even after unsuccessful builds. The default is *false*.