Docker build context puzzle

Question

My project structure:

projectRoot/
   - service/ 
     - data.json
     - Dockerfile

In that Dockerfile :

FROM node:16.14-alpine3.14

ENV THE_DATA=/tmp/data.json

COPY data.json /tmp/data.json

Under project root, if I build the image:

docker build -t service:data_tmp -f service/Dockerfile .

I get error:

 => ERROR [2/2] COPY data.json /tmp/data.json 
...
failed to compute cache key: "/data.json" not found: not found

I guess the error is due to the last . indicates build context is project root, that's why the data.json can't be located.

(My 2nd try) Then, I changed the Dockerfile to:

FROM node:16.14-alpine3.14

ENV THE_DATA=/tmp/data.json

COPY ./service/data.json /tmp/data.json

But get error:

=> ERROR [2/2] COPY ./service/data.json /tmp/data.json 
...
failed to compute cache key: "/service/data.json" not found: not found

(My 3rd try successful) I managed to make it eventually build successfully by changing the build context to /service/:

docker build -t service:data_tmp -f service/Dockerfile /service/

But I don't get why my 2nd try above is not working? I mean in my 2nd try, even though build context is still . meaning current directory, meaning project root, then, the path COPY from ./service/data.json should be correct. Why I still get error there?

Answer 1

When files that exist on the filesystem are not found in a COPY step, check two things:

1. Your context, which was done here. That's the . at the end of the build command saying the context is the current directory. If you pass a different directory, that is the source for COPY steps (at least those that don't change the source with a --from).
2. A .dockerignore file. This is in the root of the context, and has a syntax similar to .gitignore. When changing the context, you change the location docker checks for the .dockerignore file.

A common pattern for a minimal docker build is to specify the .dockerignore file with:

*
!src
# ...

Which tells docker to exclude everything on the first line, and then reinclude src on the second line. You would add additional lines to reinclude the folder here with !service.