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.

go.mod

@@ -8,7 +8,16 @@ require (
 	github.com/knadh/koanf/v2 v2.1.1
 )
 
-require github.com/gorilla/securecookie v1.1.2
+require (
+	github.com/gorilla/securecookie v1.1.2
+	github.com/spf13/pflag v1.0.5
+)
+
+require (
+	github.com/tidwall/gjson v1.14.2 // indirect
+	github.com/tidwall/match v1.1.1 // indirect
+	github.com/tidwall/pretty v1.2.0 // indirect
+)
 
 require (
 	github.com/fsnotify/fsnotify v1.7.0 // indirect
@@ -16,9 +25,11 @@ require (
 	github.com/go-viper/mapstructure/v2 v2.0.0 // indirect
 	github.com/google/uuid v1.6.0
 	github.com/knadh/koanf/maps v0.1.1 // indirect
-	github.com/knadh/koanf/providers/confmap v0.1.0
+	github.com/knadh/koanf/parsers/json v0.1.0
+	github.com/knadh/koanf/providers/posflag v0.1.0
 	github.com/mitchellh/copystructure v1.2.0 // indirect
 	github.com/mitchellh/reflectwalk v1.0.2 // indirect
+	github.com/tidwall/sjson v1.2.5
 	golang.org/x/sys v0.22.0 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )