Add an ability to specify Rust compiler version #40

Currently the CLI always uses the latest available srtool container image and thus only one Rust compiler version. It ignores rust-toolchain.toml in the root directory of the project (say there is rust-toolchain.toml and runtime/ in the project root) and there is no ability to specify what srtool container image using --image parameter because it considers the whole string as an image path and appends tag version to it.

This adds inconsistency in projects that use rust-toolchain.toml because, for example, different compiler versions are used when running cargo check on the runtime package and when releasing a build compiled using srtool-cli.