fix centos dockerfiles

Author: marcoieni

src/ci/docker/host-x86_64/dist-i686-linux/Dockerfile

@@ -2,7 +2,7 @@
 # CentOS 7 has headers for kernel 3.10, but that's fine as long as we don't
 # actually use newer APIs in rustc or std without a fallback. It's more
 # important that we match glibc for ELF symbol versioning.
-public.ecr.aws/docker/library/centos:7
+FROM public.ecr.aws/docker/library/centos:7
 
 WORKDIR /build
 

src/ci/docker/host-x86_64/dist-x86_64-linux/Dockerfile

@@ -2,7 +2,7 @@
 # CentOS 7 has headers for kernel 3.10, but that's fine as long as we don't
 # actually use newer APIs in rustc or std without a fallback. It's more
 # important that we match glibc for ELF symbol versioning.
-public.ecr.aws/docker/library/centos:7
+FROM public.ecr.aws/docker/library/centos:7
 
 WORKDIR /build