is it possible to allow failure in COPY at a Dockerfile? - docker

Question

I have a multistage Docker file like this:
From A as a
WORKDIR /app
COPY . .
From B as b
COPY --from=a /app/path/to/a/file /destination/file/path
Sometimes the source-file of last COPY does not exist and this causes a failure in docker build. Is it possbile to do the magic w/o worrying about the existence of /app/path/to/a/file?

Answer 1

I don't suppose there is a way to allow for a failure in a COPY instruction, but what you could do is copy not a specific file, but the content of the folder that does exist (even if it's empty).
For example, imagine you have a folder structure like this
.
├── Dockerfile
└── test_folder
└── test_file
1 directory, 2 files
and you build from this Dockerfile:
FROM alpine as alp1
WORKDIR /app
COPY . .
FROM alpine
RUN mkdir /dest_folder
COPY --from=alp1 /app/test_folder/test_file /dest_folder
if works because the file does exist and if we were to delete the file from that folder, the COPY would fail.
So instead of copying /app/test_folder/test_file we could copy /app/test_folder/, which will copy EVERYTHING from inside the test_folder to /dest_folder in the second container, EVEN if there is nothing:
file removed:
.
├── Dockerfile
└── test_folder
1 directory, 1 file
building from:
FROM alpine as alp1
WORKDIR /app
COPY . .
FROM alpine
RUN mkdir /dest_folder
COPY --from=alp1 /app/test_folder/ /dest_folder
> docker build -t test .
Sending build context to Docker daemon 2.56kB
Step 1/6 : FROM alpine as alp1
---> 0a97eee8041e
Step 2/6 : WORKDIR /app
---> Using cache
---> 0a6fc3a90e15
Step 3/6 : COPY . .
---> Using cache
---> 076efaa3a8b9
Step 4/6 : FROM alpine
---> 0a97eee8041e
Step 5/6 : RUN mkdir /dest_folder
---> Using cache
---> 8d647b9a1573
Step 6/6 : COPY --from=alp1 /app/test_folder/ /dest_folder
---> Using cache
---> 361b0919c585
Successfully built 361b0919c585
Successfully tagged test:latest
dest_folder exists:
docker run -it --rm test ls
bin etc media proc sbin tmp
dest_folder home mnt root srv usr
dev lib opt run sys var
but nothing is inside
docker run -it --rm test ls -lah /dest_folder
total 8K
drwxr-xr-x 1 root root 4.0K Nov 24 14:06 .
drwxr-xr-x 1 root root 4.0K Nov 24 14:13 ..