Add a test stage to run tests during the image build

Author: mikesir87

Dockerfile

@@ -37,15 +37,32 @@ COPY client/public ./public
 COPY client/src ./src
 RUN yarn build
 
+###################################################
+# Stage: test
+#
+# This stage runs the tests on the backend. This is split into a separate
+# stage to allow the final image to not have the test dependencies or test
+# cases.
+###################################################
+FROM base AS test
+COPY backend/package.json backend/yarn.lock ./
+RUN --mount=type=cache,id=yarn,target=/root/.yarn yarn install --frozen-lockfile
+COPY backend/spec ./spec
+COPY backend/src ./src
+RUN yarn test
+
 ###################################################
 # Stage: final
 #
 # This stage is intended to be the final "production" image. It sets up the
 # backend and copies the built client application from the client-build stage.
+#
+# It pulls the package.json and yarn.lock from the test stage to ensure that
+# the tests run (without this, the test stage would simply be skipped).
 ###################################################
 FROM base AS final
 ENV NODE_ENV=production
-COPY backend/package.json backend/yarn.lock ./
+COPY --from=test /usr/local/app/package.json /usr/local/app/yarn.lock ./
 RUN --mount=type=cache,id=yarn,target=/root/.yarn yarn install --production --frozen-lockfile
 COPY backend/src ./src
 COPY --from=client-build /usr/local/app/dist ./src/static