Use DEV_DOCKER constant only in docker-compose.yml file

Author: Turbo87

.env.sample

@@ -1,6 +1,3 @@
-# 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`.

docker-compose.yml

@@ -15,6 +15,7 @@ services:
       context: .
       dockerfile: backend.Dockerfile
     environment:
+      DEV_DOCKER: "true"
       DATABASE_URL: postgres://postgres:password@postgres/cargo_registry
       SESSION_KEY: badkeyabcdefghijklmnopqrstuvwxyzabcdef
       GIT_REPO_URL: file://./tmp/index-bare

docs/CONTRIBUTING.md

@@ -513,9 +513,6 @@ 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.
@@ -529,7 +526,6 @@ 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`