fix: include the prefix directory in .dockerignore

Noticed on macOS that ./scripts/build-docker.sh was failing.
Our Dockerfile runs ./scripts/setup-dependencies.sh in the container
before running `COPY . .`, which if prefix/ is not ignored,
will copy in the libraries built on the host.

Signed-off-by: Michael Maurer <maurer.mi@northeastern.edu>

.dockerignore

@@ -3,6 +3,7 @@ run
 plots
 .vscode
 .deps
+prefix
 
 # E2E test results
 testruns