Heroku support

[bklang]

This does two things:

1. It configures the app for PostgreSQL in both production (Heroku) and local (using Docker)
2. Renames an asset to work around this error:

2020-01-17T16:10:51.207654+00:00 app[web.1]: [f0f6d13a-b942-49c8-a5a6-119bd360d913] ActionView::Template::Error (The asset "plant_default.jpeg" is not present in the asset pipeline.):
2020-01-17T16:10:51.207656+00:00 app[web.1]: [f0f6d13a-b942-49c8-a5a6-119bd360d913]     34:   <% end %>
2020-01-17T16:10:51.207660+00:00 app[web.1]: [f0f6d13a-b942-49c8-a5a6-119bd360d913]     35:   <div class="col mb-4">
2020-01-17T16:10:51.207663+00:00 app[web.1]: [f0f6d13a-b942-49c8-a5a6-119bd360d913]     36:     <div class="card h-100">
2020-01-17T16:10:51.207715+00:00 app[web.1]: [f0f6d13a-b942-49c8-a5a6-119bd360d913]     37:       <%= image_tag('plant_default.jpeg', width: '100%') %>
2020-01-17T16:10:51.207719+00:00 app[web.1]: [f0f6d13a-b942-49c8-a5a6-119bd360d913]     38:       <div class="card-body">
2020-01-17T16:10:51.207721+00:00 app[web.1]: [f0f6d13a-b942-49c8-a5a6-119bd360d913]     39:         <h3 class="card-title">
2020-01-17T16:10:51.207723+00:00 app[web.1]: [f0f6d13a-b942-49c8-a5a6-119bd360d913]     40:           There is always room for more!
2020-01-17T16:10:51.207726+00:00 app[web.1]: [f0f6d13a-b942-49c8-a5a6-119bd360d913]
2020-01-17T16:10:51.207728+00:00 app[web.1]: [f0f6d13a-b942-49c8-a5a6-119bd360d913] app/views/plants/index.html.erb:37

I do not know why Heroku and/or Rails wants .jpg instead of .jpeg, but this fixed it.

To start the app for local development:

1. Run docker-compose up -d to start the Postgres container
2. Ensure you have dotenv installed (gem install dotenv)
3. Copy the sample environment file in the root of this repo: cp env .env
4. Boot up Rails: dotenv bin/rails s

[bklang]

Probable explanation for the file rename issue: rails/sprockets-rails#304