static build: dockerfile entrypoint set to exec form

According to dockerfile reference, there are two forms of specifying
entrypoint: exec and shell. Exec form is preferrable and  allows use
this image in scripts.

Close #4960

Dockerfile.staticbuild

@@ -95,4 +95,4 @@ RUN if [ -n "${RUN_TESTS}" ]; then \
         /usr/bin/python test-run.py --force; \
     fi
 
-ENTRYPOINT /bin/bash
+ENTRYPOINT ["/bin/bash"]