vagrant@localhost:~/build-from-digest docker build -t foo .
Sending build context to Docker daemon 2.048 kB
Sending build context to Docker daemon
Step 0 : FROM localhost:5000/andy/busybox@sha256:6b019df8c73bb42e606225ef935760b9c428521eba4ad2519ef3ff4cdb3dbd69
 ---> 4986bf8c1536
Step 1 : ENV a b
 ---> Running in 45397636f2d9
 ---> cac0f814f5ed
Removing intermediate container 45397636f2d9
Successfully built cac0f814f5ed