How to use Docker's COPY/ADD instructions to c

Question

I am trying to copy a single file from the root of the build context into a newly created directory in a docker image.

The Dockerfile that I am using is as follows:

FROM debian:latest
RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app
COPY test.txt /usr/src/app

The test.txt file is a simple ASCII text file as follows:

$ cat test.txt 
This is a test

However, when I build this image I get an image stating that the destination path is not a directory.

$ docker build .
Sending build context to Docker daemon 4.608 kB
Sending build context to Docker daemon 
Step 0 : FROM debian:latest
 ---> 9a61b6b1315e
Step 1 : RUN mkdir -p /usr/src/app
 ---> Using cache
 ---> 86bd44a8776e
Step 2 : WORKDIR /usr/src/app
 ---> Using cache
 ---> ed6771adc681
Step 3 : ADD test.txt /usr/src/app
stat /var/lib/docker/devicemapper/mnt/ee5b9b7029f2adf27d332cbb0d98d6ad9927629a7569fd2d9574cb767b23547b/rootfs/usr/src/app/test.txt: not a directory

I have tried using multiple combinations of docker versions, base images, and distributions with docker installed. I have also tried using ADD instead of COPY but the result is the same.

I am currently trying this on CentOS 7 with the following docker version installed:

Client version: 1.7.1
Client API version: 1.19
Go version (client): go1.4.2
Git commit (client): 786b29d
OS/Arch (client): linux/amd64
Server version: 1.7.1
Server API version: 1.19
Go version (server): go1.4.2
Git commit (server): 786b29d
OS/Arch (server): linux/amd64

What I have noticed is that I can copy a directory, but not a file. For example, if I create a "test" directory in the build context root and put test.txt inside the test directory then I can copy the directory successfully:

FROM debian:latest
RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app
COPY test /usr/src/app

Note that in the above Dockerfile I copy the entire ./test directory rather than just ./test.txt. This build successfully.

The Docker documentation has the following sample use case for the COPY instruction which is similar to what I am trying to do:

COPY hom?.txt /mydir/

Can anyone point out what I am doing wrong? How can I go about copying a single file?

Answer 1

For me, I was adding a path based on the docker-compose.yml instead of the Dockerfile:

File structure

.
..
/docker-compose.yml
/docker/
/docker/php/
/docker/php/Dockerfile
/docker/php/my_file

and in / I was running docker-compose up --build.

instead of COPY docker/php/my_file /some/location I needed to run COPY my_file /some/location

Answer 2

As stated in the Dockerfile documentation:

• If <src> is any other kind of file, it is copied individually along with its metadata. In this case, if <dest> ends with a trailing slash /, it will be considered a directory and the contents of <src> will be written at <dest>/base(<src>).

• If <dest> does not end with a trailing slash, it will be considered a regular file and the contents of <src> will be written at <dest>.

Thus, you have to write COPY test.txt /usr/src/app/ with a trailing /.