bug: docker build failed with dashboard not found

[hoffer32]

Issue description

docker build -t apisix:0.8-alpine --build-arg APISIX_VERSION=0.8 -f alpine/Dockerfile alpine

Cloning into 'incubator-apisix'...
Warning: unmatched variable LUA_LIBDIR
make: Nothing to be done for 'default'.
install -d /usr/local/apisix/dashboard
cd `mktemp -d /tmp/apisix.XXXXXX` && \
	git clone https://github.com/iresty/apisix.git && \
	cd apisix && \
	git submodule update --init --recursive && \
	cp -r dashboard/* /usr/local/apisix/dashboard
Cloning into 'apisix'...
cp: can't stat 'dashboard/*': No such file or directory
make: *** [Makefile:102: install] Error 1

Error: Build error: Failed installing.
The command '/bin/sh -c cd /tmp 	&& luarocks install apisix-${APISIX_VERSION}-0.rockspec --tree=/usr/local/apisix/deps' returned a non-zero code: 1

[membphis]

@moonming Do you have time to look at this?

[membphis]

you need to run git submodule init before you build the dashboard.