Docker COPY behaving inconsistently

Question

I would like to copy whitelisted folders from filesystem into target container.

Basically I'd like to perform "complex" operation cp -r app vendor target, where target is a Docker container.

I have simple Docker file:

FROM alpine

COPY app/* /www/
COPY vendor /www/

For reasons unknown the first command doesn't copy the app directory itself. It seems that Docker doesn't distinguish the difference between app and app/ which is quite irritating.

For testing purposes I've created following structure:

.
├── app
│   ├── foo
│   └── second
│       └── barf
├── Dockerfile
└── vendor
    └── bar

docker build -t test . && docker run -it test ash:

Result? Directory structure is not kept and parent directories not copied.

/www/
├── bar
├── barf
└── foo

docker info:

Server Version: 1.12.1
Storage Driver: overlay2
 Backing Filesystem: extfs

Can anyone explain me why Docker's COPY . /target and COPY app/* /target behaves differently? What's the motivation? Why Docker doesn't used standard semantic of UNIX cp command?

Answer 1

Generally, COPY instruction copy files and/or directories from "/app" and adds them to the container at path "/www/". If you want to have a "app" directory inside of "/www/" then your COPY instruction should looks like:

COPY /app /www/app/

You can read more about COPY instruction in documentation. Here I paste explanation of this behaviour from it:

If is a directory, the entire contents of the directory are copied, including filesystem metadata. Note:The directory itself is not copied, just its contents.

Regarding to your update:
COPY . /target and COPY app /target behaves the same. It takes entire contents from source directory ( from app or from . directory ) and copy it to the /target directory.

You can see slightly different behaviour when you use wildcards, eg. COPY app/* /www/. It copy all files from app/ to /www/ but then it treat every single directory from app/ like a source and copy its contents to /www/.

Why in Docker COPY is not implemented in the same way like it is in UNIX's cp command? I don't know, but if you want you can create pull request with own implementation :)