compile-time microoptimization

Dockerfile

@@ -5,6 +5,6 @@ COPY . /opt/app/
 RUN cd /opt/app && \
     shards
 
-RUN cd /opt/app/ && crystal build --release src/sales_backend.cr
+RUN cd /opt/app/ && crystal build --mcpu $(gcc -march=native -Q --help=target|grep march|awk '{print $2}'|head -n 1) --release src/sales_backend.cr
 
 CMD /opt/app/sales_backend