cli: fallback to environment variable

Cargo.toml

@@ -12,7 +12,7 @@ categories = ["command-line-interface", "config", "development-tools"]
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-clap = { version = "3.2.15", default-features = false, features = ["std", "derive"] }
+clap = { version = "3.2.15", default-features = false, features = ["std", "env", "derive"] }
 
 [features]
 color = ["clap/color"]

src/cli.rs

@@ -39,6 +39,7 @@ pub struct Args {
         short = 'f',
         long,
         default_value = ".env",
+        env = "VNETOD_FILE",
         help = "Change source file with environment variables"
     )]
     pub file: PathBuf,