Revert "fix: install shellcheck (#13)"

This reverts commit f8ff0b0. This
commit is not longer necessary because we resolved the issue at the root
source (by updating the Docker container for v0.12.2 to contain the
`shellcheck` binary).

Author: claymcleod

Dockerfile

@@ -3,7 +3,5 @@ WORKDIR /app
 
 COPY . .
 
-RUN apt update && apt install -y shellcheck && rm -rf /var/lib/apt/lists/*
-
 ENTRYPOINT ["sprocket"]
 CMD ["--help"]