docker,travis: fail build on any warning

.travis.yml

@@ -9,7 +9,7 @@ jobs:
         - shards update
         - shards install
       install:
-        - crystal build --error-on-warnings src/invidious.cr
+        - crystal build --warnings all --error-on-warnings src/invidious.cr
       script:
         - crystal tool format --check
         - crystal spec

docker/Dockerfile

@@ -9,7 +9,7 @@ COPY ./src/ ./src/
 # TODO: .git folder is required for building – this is destructive.
 # See definition of CURRENT_BRANCH, CURRENT_COMMIT and CURRENT_VERSION.
 COPY ./.git/ ./.git/
-RUN crystal build --static --release \
+RUN crystal build --static --release --warnings all --error-on-warnings \
 # TODO: Remove next line, see https://github.com/crystal-lang/crystal/issues/7946
     -Dmusl \
     ./src/invidious.cr