Fix 32-bit holy-build-box builds

Without this, `uname -m` could return x86_64 in a holy-build-box-32 image

https://github.com/phusion/holy-build-box/blob/master/BUILDING-32-BIT-BINARIES.md

Makefile

@@ -118,7 +118,7 @@ linux-build-box32-regular: luvi-src.tar.gz
 	rm -rf build && mkdir -p build
 	cp packaging/holy-build.sh luvi-src.tar.gz build
 	docker run -t -i --rm \
-		  -v `pwd`/build:/io phusion/holy-build-box-32:latest bash /io/holy-build.sh regular-asm
+		  -v `pwd`/build:/io phusion/holy-build-box-32:latest linux32 bash /io/holy-build.sh regular-asm
 	mv build/luvi luvi-regular-Linux_i686
 
 linux-build-box-tiny: luvi-src.tar.gz
@@ -133,7 +133,7 @@ linux-build-box32-tiny: luvi-src.tar.gz
 	rm -rf build && mkdir -p build
 	cp packaging/holy-build.sh luvi-src.tar.gz build
 	docker run -t -i --rm \
-		  -v `pwd`/build:/io phusion/holy-build-box-32:latest bash /io/holy-build.sh tiny
+		  -v `pwd`/build:/io phusion/holy-build-box-32:latest linux32 bash /io/holy-build.sh tiny
 	mv build/luvi luvi-tiny-Linux_i686
 
 publish-src: reset luvi-src.tar.gz