import: allow users to set os, arch and variant of imports

Allows users to set `--os` , `--arch` and `--variant` of the image
created from the custom import.

Following is useful when user is already aware of the values which are
correct for their generated rootfs

Signed-off-by: Aditya R <arajan@redhat.com>

docs/source/markdown/podman-import.1.md

@@ -19,6 +19,10 @@ Note: `:` is a restricted character and cannot be part of the file name.
 
 ## OPTIONS
 
+#### **--arch**
+
+Set architecture of the imported image.
+
 #### **--change**=*instruction*, **-c**
 
 Apply the following possible instructions to the created image:
@@ -30,10 +34,18 @@ Can be set multiple times
 
 Set commit message for imported image
 
+#### **--os**
+
+Set OS of the imported image.
+
 #### **--quiet**, **-q**
 
 Shows progress on the import
 
+#### **--variant**
+
+Set variant of the imported image.
+
 **--verbose**
 
 Print additional debugging information