Set no-op mandatory env vars for Docker build

Our app requires some environment variables, and will refuse to boot if
they are not set to ensure we don't bring up the app without missing
config.

Unfortunately, this means we need to manually set these in the
Dockerfile build stage too so that the assets can be built.

Dockerfile

@@ -10,7 +10,14 @@ COPY Gemfile* .ruby-version ./
 RUN bundle install
 COPY . .
 RUN bootsnap precompile --gemfile .
-RUN SECRET_KEY_BASE=none GOVUK_NOTIFY_API_KEY=none rails assets:precompile && rm -fr log
+
+# Set mandatory env vars to allow Rails to boot for asset compilation. These are only set for the
+# current build stage.
+ENV SECRET_KEY_BASE=none
+ENV GOVUK_NOTIFY_API_KEY=none
+ENV DISCOVERY_ENGINE_DATASTORE=none
+ENV DISCOVERY_ENGINE_DEFAULT_COLLECTION_NAME=none
+RUN rails assets:precompile && rm -fr log
 
 
 FROM --platform=$TARGETPLATFORM $base_image