Simple but effective. If you copy just the package.json and run npm install in one step Docker will invalidate its own cache only if the file changes.
FROM node:lts-alpine as build
WORKDIR /
# Installing `node_modules` only if package.json has changed
COPY ./package.json ./package-lock.json .
RUN npm ci
# Copy the rest and build the thing
COPY . .
RUN npm run build
FROM nginx:latest as site
COPY --from=build /public /var/www/html
ADD /nginx.conf /etc/nginx/nginx.conf
EXPOSE 80/tcp