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

@@ -606,6 +606,56 @@ 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>`
+
+  When building multi-arch manifest-lists or images for a foreign-architecture,
+  it's often helpful to have access to platform details within the `Containerfile`.
+  For example, when using a `RUN curl ...` command to install OS/Arch specific
+  binary into the image.  Or, if certain `RUN` operations are known incompatible
+  or non-performant when emulating a specific architecture.
+
+  There are several named `ARG` variables available. The purpose of each should be
+  self-evident by its name.  _However_, in all cases these ARG values are **not**
+  automatically populated.  You must always declare them within each `FROM` section
+  of the `Containerfile`.
+
+  The available `ARG <name>` variables are available with two prefixes:
+
+  * `TARGET...` variable names represent details about the currently running build
+    context (i.e. "inside" the container).  These are often the most useful:
+    * `TARGETOS`: For example `linux`
+    * `TARGETARCH`: For example `amd64`
+    * `TARGETPLATFORM`: For example `linux/amd64`
+    * `TARGETVARIANT`: Uncommonly used, specific to `TARGETARCH`
+  * `BUILD...` variable names signify details about the _host_ performing the build
+    (i.e. "outside" the container):
+    * `BUILDOS`: OS of host performing the build
+    * `BUILDARCH`: Arch of host performing the build
+    * `BUILDPLATFORM`: Combined OS/Arch of host performing the build
+    * `BUILDVARIANT`: Uncommonly used, specific to `BUILDARCH`
+
+  An example `Containerfile` that uses `TARGETARCH` to fetch an arch-specific binary could be:
+
+  ```
+  FROM busybox
+  ARG TARGETARCH
+  RUN curl -sSf -O https://example.com/downloads/bin-${TARGETARCH}.zip
+  ```
+
+  Assuming the host platform is `linux/amd64` and foreign-architecture emulation
+  enabled (e.g. `qemu-user-static`), then running the command:
+
+  ```
+  $ podman build --platform linux/s390x .
+  ```
+
+  Would end up running `curl` on `https://example.com/downloads/bin-s390x.zip` and producing
+  a container image suited for the the `linux/s390x` platform.  **Note:** Emulation isn't
+  strictly required, these special build-args will also function when building using
+  `podman farm build`.
+
+
 **ONBUILD**
   -- `ONBUILD [INSTRUCTION]`
   The **ONBUILD** instruction adds a trigger instruction to an image. The