Create Ubuntu Dockerfile for local builds

From Gavin:
```
apt update
apt install build-essential
apt install git
apt install curl
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs/ | sh
source "$HOME/.cargo/env"
git clone https://github.com/contentauth/c2pa-c.git
cd c2pa-c
cargo install cbindgen
make test
```