build: fix static build condition about testing

Before this patch RUN_TESTS condition in Dockerfile.staticbuild was
ignored and always was true.

However adding of brackets solves only part of problem. If RUN_TESTS is
empty `sh -c` returns 1 and build fails.

However if we run tests we should fail build if tests are not passed.
Ternary logic was rewritten to fair if-else.

This patch fixes it and allows build tarantool statically without
running tests.

@Totktonada: Fixed .gitlab.mk to pass RUN_TESTS environment variable to
docker build arguments.

Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>

.gitlab.mk

@@ -111,4 +111,4 @@ package: git_submodule_update
 # ############
 
 static_build:
-	docker build --network=host -f Dockerfile.staticbuild .
+	docker build --network=host --build-arg RUN_TESTS="${RUN_TESTS}" -f Dockerfile.staticbuild .

Dockerfile.staticbuild

@@ -90,9 +90,10 @@ RUN set -x \
 RUN cd /tarantool && make install
 
 ARG RUN_TESTS
-RUN [ -n ${RUN_TESTS} ] && \
-    set -x && \
-    cd test && \
-    /usr/bin/python test-run.py --force
+RUN if [ -n "${RUN_TESTS}" ]; then \
+        set -x && \
+        cd test && \
+        /usr/bin/python test-run.py --force; \
+    fi
 
 ENTRYPOINT /bin/bash