fix: substitute devcontainer.json build args into Dockerfile ARG variables#490
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fix: substitute devcontainer.json build args into Dockerfile ARG variables#490
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…ables (#455) ImageFromDockerfile and UserFromDockerfile now accept external build args from devcontainer.json, allowing them to override ARG defaults and supply values for ARGs without defaults. This fixes the error when a Dockerfile uses ARG VARIANT + FROM ...${VARIANT} with the value provided in devcontainer.json build.args.
johnstcn
reviewed
Feb 18, 2026
- Change buildArgs parameter from []string to map[string]string - Add exported BuildArgsMap helper for call-site conversion - Use child.Next.Value instead of fragile child.Original slicing - Use strings.Cut instead of strings.Contains + SplitN - Return ProcessWord errors instead of silently swallowing them - Add UserFromDockerfile_BuildArgs test with registry - Add MissingBuildArgUsesEmpty test case
johnstcn
reviewed
Feb 18, 2026
devcontainer/devcontainer_test.go
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One point I'd like to raise here: if you try to build this Dockerfile it will probably error with something like
ERROR: failed to build: failed to solve: failed to parse stage name "python:1-": invalid reference format
whereas a different Dockerfile would probably not fail e.g.
FROM alpine:3.18
ARG VARIANT
RUN echo "foo-${VARIANT}" > /tmp/test.txt && cat /tmp/test.txt
EOF
Docker also seems to have some kind of "special-case" warning about this e.g.
�[0m - InvalidDefaultArgInFrom: Default value for ARG alpine:${VARIANT} results in empty or invalid base image name (line 2)
The test asserted that an ARG without a default used in FROM would produce an empty substitution (e.g. python:1-), but in practice Docker would error on such an invalid reference. Remove the unrealistic test case.
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Fixes #455
Problem
When
devcontainer.jsonspecifies build args (e.g."VARIANT": "3.11-bookworm") and the Dockerfile usesARG VARIANT+FROM ...${VARIANT}, envbuilder fails becauseImageFromDockerfileandUserFromDockerfiledon't accept external build args — they only use ARG defaults from the Dockerfile itself.Changes
ImageFromDockerfile— AddedbuildArgs []stringparameter. Build args now override ARG defaults and supply values for ARGs without defaults.UserFromDockerfile— AddedbuildArgs []stringparameter. Collects ARG values with build arg overrides and substitutes them intostage.BaseNamebefore parsing image refs.