Require ?st= hash with PUBLISH_SECRET for publish.

 - Use PUBLISH_SECRET environment variable as secret key component
   of secure_link publish gate.

 - Publishing is not allowed without the correct st parameter.
   The st parameter can be obtained with the following command
   where `$stream_name` is the name of the stream being published:

    echo -n '${PUBLISH_SECRET} $stream_name' | \
        openssl md5 -binary | openssl base64 | tr +/ -_ | tr -d =

 - Supply the PUBLISH_SECRET when starting the container. For example,
   start the docker container with `-e PUBLISH_SECRET=value` to set
   the key to `value`.
Run ldconfig(8) to setup libfdk-aac for avconv.
Push to player to allow dynamic stream names.

Pulling from the source in player requires knowing the stream name in
advance. Use push to allow for the publishers to use multiple streams
that can use the same player application.
Bust Docker cache on nginx.conf.tmpl modification.

Using ADD on a directory causes Docker builds to skip any attempt at
checking if directory contents were modified since the last run.

Since we want to bust cache when we modifiy the configuration,
explicitly define the configuration file to allow Docker to check if the
file was modified.

We may need to do this for sbin files in the future, too.
Expose ports earlier for better cache use.
Use PUBLISH_SECRET in pub app name. Hide stats.

 - Our Teradek VidiU + nginx-rtmp setup is not able to provide the `st`
   parameter as a $arg_st to nginx. This could be a limitation of
   the nginx-rtmp on_publish directive.

 - Instead of using `st`, just put the PUBLISH_SECRET as the publish
   application name. The encoder should publish to
   `/pub_${PUBLISH_SECRET}/$name`, where $name is the stream desired.

 - Public stats would leak the PUBLISH_SECRET, so we need to secure
   these links. Do so with the secure_links module. The old /info page
   may be accessed by visiting `/p/$token/info`, where $token is the
   result of:

     echo -n 'info${PUBLISH_SECRET}' | openssl md5 -hex

   The same format can be used for `/p/$token/stats`, replacing info
   above with stats.
Show STANDBY_IMAGE_URL for HLS if stream offline.

 - Explictly ADD sbin files to bust Docker cache when they are modified.

 - Download STANDBY_IMAGE_URL inside the entrypoint.

 - Show the standby image by attempting to publish a the image as a FLV
   stream. This publish will fail if the real stream is being published.
Add documentation.