feat: environment configuration

Environment variables can now be used to configure any
config property including the feeds list.

This makes it easier to use in environments without access
to config files like GCR. Some may prefer not to have a separate
config file as well.

Also added build metadata to the docker image and binaries.

README.md

@@ -22,6 +22,8 @@ By running your own OPDS Proxy you can allow eReaders to navigate and download b
 
 You can run OPDS Proxy as a Docker container or as an executable.
 
+Configuration is done via YAML file (default `config.yml`), environment variables, and command flags in that order of precedence.
+
 ### Docker 
 
 Docker images are published to [Docker Hub](https://hub.docker.com/r/evanbuss/opds-proxy) and the [GitHub Container Registry](https://github.com/evan-buss/opds-proxy/pkgs/container/opds-proxy).
@@ -32,6 +34,14 @@ services:
     image: evanbuss/opds-proxy:latest
     #image: ghcr.io/evan-buss/opds-proxy:latest
     container_name: opds-proxy
+    # You can also use environment variables to configure the container
+    # environment:
+    #   - OPDS__PORT=5228
+    #   - OPDS__FEEDS__0__NAME=Some Feed
+    #   - OPDS__FEEDS__0__URL=http://some-feed.com/opds
+    #   - OPDS__FEEDS__0__AUTH__USERNAME=user
+    #   - OPDS__FEEDS__0__AUTH__PASSWORD=password
+    #   - OPDS__FEEDS__0__AUTH__LOCAL_ONLY=true
     ports:
       - 8080:8080
     volumes: