gistfile1.txt

 # cat <<EOT > Dockerfile
heredoc> from busybox
heredoc> copy foo /
heredoc> EOT
 # echo "bar" > foo
 # docker build .
Sending build context to Docker daemon 4.096 kB
Step 1/2 : FROM busybox
---> 7968321274dc
Step 2/2 : COPY foo /
---> 90c37559c514
Removing intermediate container f2ae96b9beb4
Successfully built 90c37559c514
 # ln foo abc
 # docker build .
Sending build context to Docker daemon 3.584 kB
Step 1/2 : FROM busybox
 ---> 7968321274dc
Step 2/2 : COPY foo /  <<<<<<-------------  NO CACHE
 ---> 063c39051fdc
Removing intermediate container 0b266d615123
Successfully built 063c39051fdc
 # echo "new data" >> foo
 # docker build .
Sending build context to Docker daemon 3.584 kB
Step 1/2 : FROM busybox
 ---> 7968321274dc
Step 2/2 : COPY foo /
 ---> Using cache      <<<<<<-------------  USING CACHE
 ---> 063c39051fdc
Successfully built 063c39051fdc