dockerfile: use x86-64-v2 instead to support more CPUs

I used x86-64-v3 because my CPU supports it, but I doubt this is going
to make Invidious any faster of most of the operations.

docker/Dockerfile

@@ -23,7 +23,7 @@ RUN crystal spec --warnings all \
     --link-flags "-lxml2 -llzma"
 RUN if [[ "${release}" == 1 ]] ; then \
         crystal build ./src/invidious.cr \
-        --release --mcpu=x86-64-v3 \
+        --release --mcpu=x86-64-v2 \
         --static --warnings all \
         --link-flags "-lxml2 -llzma"; \
     else \