Add containerised build instructions

Cargo.toml

@@ -7,8 +7,7 @@ authors = ["jellywx <judesouthworth@pm.me>"]
 edition = "2018"
 
 [dependencies]
-songbird = { git = "https://github.com/Erk-/songbird", branch = "do-not-fail-if-new-opcode", features = ["builtin-queue"] }
-# songbird = { version = "0.3", features = ["builtin-queue"] }
+songbird = { version = "0.3", features = ["builtin-queue"] }
 poise = "0.3"
 sqlx = { version = "0.5", default-features = false, features = ["runtime-tokio-rustls", "macros", "mysql", "bigdecimal", "migrate"] }
 tokio = { version = "1", features = ["fs", "process", "io-util"] }