Let's say I have a folder structure similar to the following:

.
├── pants.toml
└── services
    ├── base_images
    │   ├── service_a
    │   │   ├── BUILD
    │   │   ├── Dockerfile
    │   │   ├── file_a1.txt
    │   │   ├── file_a2.txt
    │   │   └── file_a3.txt
    │   └── service_b
    │       ├── BUILD
    │       ├── Dockerfile
    │       └── file_b1.txt
    └── core
        └── service_c
            ├── BUILD
            ├── Dockerfile
            ├── file_c1.txt
            └── file_c2.txt

In this example we have three Dockerfiles that live inside a nested folder structure (in reality I have a bunch more but this simplified example should illustrate the point).

The various files

file_*.txt

represent files that need to be copied into the docker images via

COPY

instructions.

In order to avoid lengthy paths in those

COPY

instructions, I'd like to keep the

context_root

for each docker image in the folder where the respective Dockerfile lives. I.e., I'd like to be able to write something like

COPY file_a1.txt file_a2.txt file_a3.txt .

instead of

COPY services/base_images/service_a/file_a1.txt services/base_images/service_a/file_a2.txt services/base_images/service_a/file_a3.txt .

(Admittedly in this simple example we could use globs instead of listing all filenames explicitly, but in my real example there is no relation between the filenames.)

However, the complication is that the docker image for

service_b

is derived from the one for

service_a

, and similarly for

service_c

and

service_b

.

My understanding is that in order to tell pants about these dependencies, the BUILD files needs to look similar to this (with the context root at the toplevel of the repository).

$ cat services/base_images/service_a/BUILD

docker_image(name="base-image-a")

$ cat services/base_images/service_b/BUILD

docker_image(
  name="base-image-b",
  dependencies=[
    "services/base_images/service_a:base-image-a",
  ],
)

Is there any way to keep the context roots at subfolders containing the Dockerfiles while still being able to specify the dependencies between docker images?

Many thanks for reading this whole thread and for any suggestions!

(I'm also happy to provide a minimal example repo if it would be useful.)

To answer my own question: the approach described at https://www.pantsbuild.org/2.21/docs/docker#dependency-inference-support works as expected:

# services/base_image/service_b/Dockerfile

ARG BASE_IMAGE=services/base_images/service_a:base-image-a
FROM $BASE_IMAGE

This works even when I declare

docker_image

targets with

context_root = "./"

. My confusion yesterday stemmed from the fact that in my initial attempts I had explicitly declared the docker image dependencies explicitly in the

docker_image

declaration, for example:

docker_image(name="base-image-b", dependencies=["services/base_images/service_a:base-image-a"])

. This approach requires the context root to be a common parent because now the dependency needs to be within the docker context. By contrast, when using the

ARG

approach described in the docs, the dependency is resolved by pants itself without Docker having to know about it. 👌