[UNTRACKED] Allow users of this repository as a submodule to override…

… the foundry version to use (#55)

## Summary
Allow users of this repository as a submodule to override the foundry
version to use

## Detail
This repository is used as a submodule elsewhere to build evm contracts,
at the same time these other repositories install said contracts using a
locally managed foundry version. when the versions don't align the
builds can sometimes fail causing the need to cascade update the foundry
version everywhere.

Allowing the Dockefile to use a build argument means other downstream
repositories can update the version at their own pace without having to
update this repository first.

## Testing
covered by existing tests

## Documentation

**Story:** [UNTRACKED](https://circlepay.atlassian.net/browse/)

Dockerfile

@@ -1,6 +1,7 @@
 # Use fixed foundry image
+ARG FOUNDRY_VERSION=nightly-4a8c7d0e26a1befa526222e22737740f80a7f1c5
 
-FROM ghcr.io/foundry-rs/foundry:nightly-4a8c7d0e26a1befa526222e22737740f80a7f1c5
+FROM ghcr.io/foundry-rs/foundry:${FOUNDRY_VERSION}
 
 # Copy our source code into the container
 WORKDIR /app