Add instructions on how to build from a local Docker image

I have often needed to build a Apptainer container from a local Docker image (not from a remote image in a Docker registry). I think that this might be a relatively popular use-case, but currently this workaround is only described in an issue on GitHub (apptainer/singularity#1537 (comment)).

It would be nice to also have it in the official docs.

build_a_container.rst

@@ -73,6 +73,17 @@ them into {Singularity} containers.
 
    $ sudo singularity build lolcow.sif docker://sylabsio/lolcow
 
+*************************************************
+Building from an existing local Docker container
+*************************************************
+
+You can also use ``build`` to create a {Singularity} container
+from a local Docker image.
+
+.. code::
+
+   $ sudo singularity build lolcow.sif docker-daemon://lolcow:latest
+
 .. _create_a_writable_container:
 
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