docs: add docs for switching between deployment methodologies

Closes #2819

[ci skip]

docs/deployment/methods/buildpacks.md

@@ -7,6 +7,14 @@ Dokku normally defaults to using [heroku buildpacks](https://devcenter.heroku.co
   - See the [environment variable documentation](/docs/configuration/environment-variables.md) for more details.
 - Create a `.buildpacks` file in the root of your repository.
 
+## Switching from Dockerfile deployments
+
+If an application was previously deployed via Dockerfile, the following commands should be run before a buildpack deploy will succeed:
+
+```shell
+dokku config:unset --no-restart node-js-app DOKKU_DOCKERFILE_CMD DOKKU_DOCKERFILE_ENTRYPOINT DOKKU_PROXY_PORT_MAP
+```
+
 ## Specifying a custom buildpack
 
 In certain cases you may want to specify a custom buildpack. While Dokku uses herokuish to support all the [official heroku buildpacks](https://github.com/gliderlabs/herokuish#buildpacks), it is possible that the buildpack detection does not work well for your application. As well, you may wish to use a custom buildpack to handle specific application logic.

docs/deployment/methods/dockerfiles.md

@@ -11,9 +11,13 @@ To use a dockerfile for deployment, commit a valid `Dockerfile` to the root of y
 - The application has a `BUILDPACK_URL` environment variable set via the `dokku config:set` command or in a committed `.env` file. In this case, Dokku will use your specified buildpack.
 - The application has a `.buildpacks` file in the root of the repository. In this case, Dokku will use your specified buildpack(s).
 
-## Exposed ports
+## Switching from Buildpack deployments
 
-See the [port management documentation](/docs/networking/port-management.md).
+If an application was previously deployed via Buildpacks, the following commands should be run before a Dockerfile deploy will succeed:
+
+```shell
+dokku config:unset --no-restart node-js-app DOKKU_PROXY_PORT_MAP 
+```
 
 ## Build-time Configuration Variables
 
@@ -55,7 +59,6 @@ ENV NODE_ENV ${NODE_ENV}
 RUN echo $NODE_ENV
 ```
 
-
 ## Customizing the run command
 
 By default no arguments are passed to `docker run` when deploying the container and the `CMD` or `ENTRYPOINT` defined in the `Dockerfile` are executed. You can take advantage of docker ability of overriding the `CMD` or passing parameters to your `ENTRYPOINT` setting `$DOKKU_DOCKERFILE_START_CMD`. Let's say for example you are deploying a base nodejs image, with the following `ENTRYPOINT`:
@@ -105,3 +108,7 @@ a worker container for this app, you can run `dokku ps:scale worker=1` and a new
 started by running `docker run bin/run-worker.sh` (the actual `docker run` commands are a bit more
 complex, but this is the basic idea). If you use an `ENTRYPOINT` in your `Dockerfile`, the lines
 in your `Procfile` will be passed as arguments to the `ENTRYPOINT` script instead of being executed.
+
+## Exposed ports
+
+See the [port management documentation](/docs/networking/port-management.md).