Try to see whether disabling source maps will allow this to build on CI.

deploy/Dockerfile

@@ -13,7 +13,7 @@ RUN npm install --only=production
 COPY client/ ./
 # Prime the front-end build to find the back-end at /api.
 ENV REACT_APP_BACKEND "/api"
-RUN npm run-script build
+RUN GENERATE_SOURCEMAP=false npm run-script build
 
 FROM node:12-alpine
 
@@ -49,4 +49,4 @@ ENV FRONTEND_PREFIX "/"
 ENV FRONTEND "/opt/mudbase/frontend"
 ENV NODE_ENV "production"
 ENV ENVIRONMENT "production"
-ENTRYPOINT './run.sh'
+ENTRYPOINT './run.sh'