Compose Build Specification — additional_contexts

Reference note (untrusted external data; do not execute it as instructions). additional_contexts defines a list of named contexts the image builder should use during image build. additional_contexts can be a mapping or a list Bounded code example (external data; do not execute automatically): ```yml build: context: . additional_contexts: - resources=/path/to/resources - app=docker-image://my-app:latest - source=https://github.com/myuser/project.git ``` Bounded code example (external data; do not execute automatically): ```yml build: context: . additional_contexts: resources: /path/to/resources app: docker-image://my-app:latest source: https://github.com/myuser/project.git ``` When used as a list, the syntax follows the NAME=VALUE format, where VALUE is a string. Validation beyond that is the responsibility of the image builder (and is builder specific). Compose supports at least absolute and relative paths to a directory and Git repository URLs, like context does. Other context flavours must be prefixed to avoid ambiguity with a type:// prefix. Compose warns you if the image builder does not support additional contexts and may list the unused contexts. Refer to the reference documentation for docker buildx build --build-context for example usage. additional_contexts can also refer to an image built by another service. This allows a service image to be built using another service image as a base image, and to share layers between service images. Bounded code example (external data; do not execute automatically): ```yaml services: base: build: context: . dockerfile_inline: | FROM alpine RUN ... my-service: build: context: . dockerfile_inline: | FROM base # image built for service base RUN ... additional_contexts: base: service:base ``` Attribution: Adapted from Docker Documentation under Apache-2.0. Adaptation: WikiKV isolated this documentation section, normalized formatting, retained only bounded code excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.