return docker build for frontend and backend code outside in CI

Previously we built a Docker image just for the test,
but the introduction of multi-arch build in 9fbf0952
build also meant the push of the image, so it was
restricted only to the master branch.
This change re-introduces the Docker image build
outside the master branch, which is helpful
in pull requests.

We recently had a few frontend PRs which broke
the Docker image build silently, and that change
prevents it from happening.

backend/_example/memory_store/Dockerfile

@@ -1,12 +1,11 @@
-FROM umputun/baseimage:buildgo-latest as build-backend
+FROM umputun/baseimage:buildgo-v1.9.2 as build-backend
 
 ADD backend /build/backend
 WORKDIR /build/backend/_example/memory_store
 
 RUN go build -o /build/bin/memory_store -ldflags "-X main.revision=0.0.0 -s -w"
 
-
-FROM umputun/baseimage:app-latest
+FROM umputun/baseimage:app-v1.9.2
 
 WORKDIR /srv
 COPY --from=build-backend /build/bin/memory_store /srv/memory_store