More customization of runtime image

I'd like to be able to customize the runtime image further. Would you be interested in accepting a PR that modifies the `Dockerfile-release.eex` file like so (with appropriate changes to the generator app)?

```diff
################################################################################
## 2. Application runtime image
## From the OS base image, installs the release built above, and sets the
## entrypoint. This is the final image executed by Google App Engine.

FROM <%= @os_image %>

+ ## If your application needs additional steps when constructing the runtime image,
+ ## use the `runtime_image_directives` option to specify them
+ <%= for directive <- @runtime_image_directives do %>
+ <%= directive %>
+ <% end %>
- ...

## Copy built release from build stage into /app
COPY --from=app-build /app/_build/<%= @mix_env %>/rel/<%= @release_app %> /app/

## Command to start the release.
CMD <%= @entrypoint %>
```