chore: added #![forbid(unsafe_code)] to the library (#722)

The motivation is to make
[`cargo-geiger`](https://github.com/geiger-rs/cargo-geiger) be sure that
there is no unsafe code

testcontainers/src/lib.rs

@@ -2,6 +2,7 @@
 #![warn(rust_2018_idioms)]
 #![cfg_attr(docsrs, deny(rustdoc::broken_intra_doc_links))]
 #![cfg_attr(docsrs, feature(doc_cfg))]
+#![forbid(unsafe_code)]
 
 //! A library for integration testing against docker containers from within Rust.
 //!