dev: support DEV_DOCKER env var and document usage

Author: Steve Manuel

.env.sample

@@ -1,3 +1,6 @@
+# Toggle for server configuration in development when using Docker. 
+export DEV_DOCKER=1
+
 # Location of the *postgres* database. For example, if you have created a
 # blank database locally named `cargo_registry`, this would be
 # `postgres://postgres@localhost/cargo_registry`.

docs/CONTRIBUTING.md

@@ -513,6 +513,9 @@ one go:
 docker-compose up -d
 ```
 
+> **Note:** set `DEV_DOCKER=1` in a `.env` or `docker-compose.override.yml` 
+file when working with Docker in development.
+
 The Compose file is filled out with a sane set of defaults that should Just
 Work™ out of the box without any modification. Individual settings can be
 overridden by creating a `docker-compose.override.yml` with the updated config.
@@ -526,8 +529,12 @@ services:
     environment:
       GH_CLIENT_ID: blahblah_ID
       GH_CLIENT_SECRET: blahblah_secret
+      DEV_DOCKER: 1
 ```
 
+These environment variables can also be defined in a local `.env` file, see `.env.sample`
+for various configuration options. 
+
 #### Accessing services
 
 By default, the services will be exposed on their normal ports:

src/bin/server.rs

@@ -60,7 +60,13 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     let heroku = dotenv::var("HEROKU").is_ok();
     let fastboot = dotenv::var("USE_FASTBOOT").is_ok();
+    let dev_docker = dotenv::var("DEV_DOCKER").is_ok();
 
+    let ip = if dev_docker {
+        [0, 0, 0, 0]
+    } else {
+        [127, 0, 0, 1]
+    };
     let port = if heroku {
         8888
     } else {
@@ -103,7 +109,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
                 async move { Service::from_blocking(handler, addr) }
             });
 
-        let addr = ([0, 0, 0, 0], port).into();
+        let addr = (ip, port).into();
         #[allow(clippy::async_yields_async)]
         let server = rt.block_on(async { hyper::Server::bind(&addr).serve(make_service) });