move 'make check' into Dockerfile, simplify CI to just 'docker build .'

Per reviewer feedback: the Dockerfile now runs 'make check' early in
its build process (after dependencies are extracted). The CI workflow
is simplified to just run 'docker build .', which implicitly runs
'make check' as part of the Docker build.

Dockerfile

@@ -17,6 +17,7 @@ ARG DRONE_COMMIT_SHA unknown
 RUN mkdir -p "$(go env GOMODCACHE)" && cd "$(go env GOMODCACHE)" && \
     zstdmt -d --stdout /build/modcache.tzst | tar xf - && \
     rm /build/modcache.tzst && cd /build
+RUN make check
 RUN \
     cd mfer && go generate . && cd .. && \
     GOPACKAGESDEBUG=true golangci-lint run ./... && \