Mention the contracts Docker image in the project README (use-ink#323)

* Mention the contracts Docker image in the project README

* Add wording suggestion

Co-authored-by: Michael Müller <michi@parity.io>

* Add line between header and text start

Co-authored-by: Michael Müller <michi@parity.io>

README.md

@@ -45,12 +45,17 @@ More relevant links:
   * [Homebrew](https://formulae.brew.sh/formula/binaryen): `brew install binaryen`
   * [Arch Linux](https://archlinux.org/packages/community/x86_64/binaryen/): `pacman -S binaryen`
   * Windows: [binary releases are available](https://github.com/WebAssembly/binaryen/releases)
-    
+
   There's only an old version in your distributions package manager? Just use a 
   [binary release](https://github.com/WebAssembly/binaryen/releases).
-  
+
 * Step 3: `cargo install --force cargo-contract`
 
+### Docker Image
+
+If you'd prefer to use Docker instead we have a Docker image [available on the Docker Hub](https://hub.docker.com/r/paritytech/contracts-ci-linux)
+which contains the dependencies needed to build `cargo-contract`, as well as
+a pre-built `cargo-contract` binary which you can just execute.
 
 ## Usage
 
@@ -97,4 +102,3 @@ The entire code within this repository is licensed under the [GPLv3](LICENSE).
 
 Please [contact us](https://www.parity.io/contact/) if you have questions about
 the licensing of our products.
-