feat: read settings from environment variables

Cargo.toml

@@ -14,6 +14,7 @@ tower-http = { version = "0.6", features = ["compression-gzip", "cors", "trace",
 reqwest = { version = "0.13", features = ["json", "gzip"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+serde_path_to_error = "0.1"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter", "json"] }
 thiserror = "2"

README.md

@@ -83,7 +83,8 @@ docker run --rm -p 8000:8000 edge-proxy-rs-local
 
 ## Configuration
 
-Edge Proxy expects to load configuration from `./config.json`.
+Edge Proxy loads configuration from `./config.json` and environment variables. Environment variables take precedence, and
+the config file is optional.
 
 ### Configuration Options
 
@@ -125,6 +126,21 @@ docker run \
     flagsmith/edge-proxy-rs:latest
 ```
 
+### Settings from Environment Variables
+
+Any top-level setting can be set with an environment variable of the same name, case-insensitive. String settings take
+the value as-is; all others take JSON. An environment variable replaces the whole setting, so
+`LOGGING='{"log_level":"DEBUG"}'` resets the other logging settings to their defaults. Empty values are ignored.
+
+```bash
+docker run \
+    -e ENVIRONMENT_KEY_PAIRS='[{"server_side_key":"ser.YOUR_SERVER_KEY","client_side_key":"YOUR_CLIENT_KEY"}]' \
+    -e API_POLL_FREQUENCY_SECONDS=30 \
+    -e ALLOW_ORIGINS='["https://app.example.com"]' \
+    -p 8000:8000 \
+    flagsmith/edge-proxy-rs:latest
+```
+
 ### Environment Variables
 
 | Variable | Description | Default |