But I went into this bug:

• When I first mount a directory into a container, everything works fine.
• After I change some code, the container aborted with a syntax error.
• Then I attach into the container and see what happened.
  • When I use vim to open the "syntax error" file, everything seems to be fine.
  • When I use cat to get the file content. Then change I made to this file appeared at the end of this file instead of the place I wanted it to be.

After some search, it seems to be a virtualbox bug:

• File mount does not update with changes from host · Issue #15793 · docker/docker
  • #14920 (VboxSF volumes mounted in Docker containers do not update file contents) – Oracle VM VirtualBox
  • #9069 (shared folder doesn't seem to update) – Oracle VM VirtualBox

But this bug may not appear on everyone's machine. At least in this post¹, the speaker can get the file syncing correctly, I don't know if this is due to she didn't do any deletion. (If you are also interested in developing in Docker, you should read this post too)

The solution is simple:

Just don't use virtualbox as the docker running virtual machine. Use xhyve instead, which is also the VM used by the new Docker for Mac². You can also check out this post³ about Docker for Mac.

And here is my post⁴ about using xhyve as the Docker client host VM.