Docker build now also uses BUILD_TYPE=release. (#712)

The dockerignore and dockerfile have also been excluded from being moved into
docker images, saving docker layer cache busts if only those are changed.

.dockerignore

@@ -12,3 +12,7 @@ test_output
 .zenith
 integration_tests/.zenith
 .mypy_cache
+
+Dockerfile
+.dockerignore
+

Dockerfile

@@ -10,6 +10,7 @@ FROM zenithdb/build:buster AS pg-build
 WORKDIR /zenith
 COPY ./vendor/postgres vendor/postgres
 COPY ./Makefile Makefile
+ENV BUILD_TYPE release
 RUN make -j $(getconf _NPROCESSORS_ONLN) -s postgres
 RUN rm -rf postgres_install/build