Dockerfile: Fix error while running make server (pingcap#1361)

The `LDFLAGS` in Makefile tries to get git commit hash while building
server component, but default COPY command does not put *.git* directory
into docker container, which causes the fatal error while executing
command `git rev-parse HEAD`.

This fix remove *.git* directory from *.dockerignore* file, then the git
command can be executed successfully in container.

.dockerignore

@@ -1 +0,0 @@
-.git