Create /var/lib/pixa directory in Docker image for database/cache

- Change default StateDir from ./data to /var/lib/pixa (proper Unix convention)
- Create directory owned by pixad user in Dockerfile
- Set WORKDIR to /var/lib/pixa

internal/config/config.go

@@ -17,7 +17,7 @@ import (
 // Default configuration values.
 const (
 	DefaultPort                       = 8080
-	DefaultStateDir                   = "./data"
+	DefaultStateDir                   = "/var/lib/pixa"
 	DefaultUpstreamConnectionsPerHost = 20
 )