Docs: Semiautomatic platform ARGs

Available for quite some time, but often useful and undocumented.
Fix that and a minor whitespace problem.

Signed-off-by: Chris Evich <cevich@redhat.com>

docs/Containerfile.5.md

@@ -142,7 +142,7 @@ Current supported mount TYPES are bind, cache, secret and tmpfs.
 	      The relabel=private and Z options tell the engine to label the content with a private unshared label. Only the current container can use a private mount.
 
 	      Relabeling walks the file system under the mount and changes the label on each file, if the mount has thousands of inodes, this process takes a long time, delaying the start of the container.
-	      
+
 	      · rw, read-write: allows writes on the mount.
 
        Options specific to tmpfs:
@@ -606,6 +606,38 @@ The secret needs to be passed to the build using the --secret flag. The final im
   $ podman build --build-arg HTTPS_PROXY=https://my-proxy.example.com .
   ```
 
+Platform/OS/Arch **ARG**
+   -- ARG <name>
+
+  Additional semiautomatic predefined `ARG` variables are available to
+  assist when producing multi-arch manifest-lists or foreign-architecture
+  images.  The purpose of each of of these variables should mostly be
+  evident from their names.  Where the "BUILD" prefix signifies details
+  about the host performing the build, and the "TARGET" prefix represents
+  the intended platform for the content.
+
+  Each of the following ARG variables must be defined empty before use,
+  within each `FROM` section of the `Containerfile`.  If not defined, they
+  will _not_ be populated with values at build time.  Not all variables
+  are applicable to every OS and/or ARCH (for example `VARIANT`).
+
+  * `BUILDPLATFORM`: for example "linux/amd64"
+  * `BUILDOS`: for example "linux"
+  * `BUILDARCH`: for example "amd64"
+  * `BUILDVARIANT`: uncommonly used, specific to arch.
+  * `TARGETPLATFORM`
+  * `TARGETOS`
+  * `TARGETARCH`
+  * `TARGETVARIANT`
+
+  Example `Containerfile`:
+
+  ```
+  FROM busybox
+  ARG TARGETARCH
+  RUN curl -sSf -O https://example.com/app/downloads/foo-${TARGETARCH}.zip
+  ```
+
 **ONBUILD**
   -- `ONBUILD [INSTRUCTION]`
   The **ONBUILD** instruction adds a trigger instruction to an image. The