[Dockerfile] set env CHECK_SKIPS_FUNCTIONAL_TEST=true for docker test

this is set by the traditional test job, and should be set for the
docker build, too.

Author: notandy

internal/dockerfile/Dockerfile.tmpl

@@ -55,6 +55,7 @@ RUN {{ range $dcfg.CheckEnv }}{{ . }} {{ end }}make -C /src static-check
 # but for it to be able to read the go cache, we need to allow it.
 RUN chown -R 4200:4200 /src/ /go/
 USER 4200:4200
+ENV CHECK_SKIPS_FUNCTIONAL_TEST=true
 RUN cd /src \
   && git config --global --add safe.directory /src \
   && {{ range $dcfg.CheckEnv }}{{ . }} {{ end }}make build/cover.out