fix: set default base image in Dockerfiles and improve syntax handling by Kaniska244 · Pull Request #1271 · devcontainers/cli

Kaniska (Kaniska244) · 2026-07-28T09:08:19Z

What this PR does

Related #1229. Fixes the InvalidDefaultArgInFrom build warning emitted by Docker/BuildKit when building dev containers. The warning is raised whenever a FROM instruction references a build ARG that has no in-scope default value at parse time.

 1 warning found (use docker --debug to expand):
 - InvalidDefaultArgInFrom: Default value for ARG $BASE_IMAGE results in empty or invalid base image name (line 2)
[303440 ms] Start: Run: docker events --format {{json .}} --filter event=start
[303442 ms] Start: Starting container

This warning does not come from the user's project Dockerfile — it originates from the internal Dockerfiles the CLI generates (and the static updateUID.Dockerfile), where FROM referenced _DEV_CONTAINERS_BASE_IMAGE / BASE_IMAGE without a declared default.

Root cause

Two patterns were triggering the linter:

No default on the ARG — scripts/updateUID.Dockerfile declared ARG BASE_IMAGE (no default) and then used FROM $BASE_IMAGE.
Shell-style fallback that the linter doesn't credit — the generated feature Dockerfiles used FROM ${_DEV_CONTAINERS_BASE_IMAGE:-scratch}. The ${VAR:-scratch} form is a runtime shell expansion; it is not recognized as an ARG default by the static linter, so the warning persisted even when a global ARG ... = placeholder was declared.

The fix

Declare the base-image ARG with a real default (scratch for the generated files, placeholder for the UID script) immediately before it's used, and reference it plainly as $VAR — the form the linter recognizes as having a valid default.

src/spec-configuration/containerFeaturesConfiguration.ts — add ARG _DEV_CONTAINERS_BASE_IMAGE=scratch before the FROM stages in the generated feature base Dockerfile (the FROM lines already reference the ARG directly).
src/spec-node/containerFeatures.ts — change the prefix ARG default from placeholder to scratch in both getImageBuildOptions (no-features path) and getFeaturesBuildOptions (features path).
scripts/updateUID.Dockerfile — change ARG BASE_IMAGE → ARG BASE_IMAGE=placeholder.

Behavior is unchanged at build time

The CLI always passes the real image via --build-arg (_DEV_CONTAINERS_BASE_IMAGE=<image>, and BASE_IMAGE=<image> for the UID Dockerfile), so the scratch / placeholder defaults only ever exist to satisfy the static linter and never affect the actual build.

Tests

Added a daemon-free unit test suite in src/test/container-features/generateFeaturesConfig.test.ts (validate generated Dockerfiles avoid InvalidDefaultArgInFrom) that:

Introduces a findFromArgsWithoutDefault() helper mimicking the InvalidDefaultArgInFrom rule (flags any ARG used in a FROM without a default declared before it).
Asserts the generated feature base Dockerfile declares _DEV_CONTAINERS_BASE_IMAGE with a default, references it directly, and does not use the ${VAR:-default} shell fallback.
Asserts scripts/updateUID.Dockerfile declares BASE_IMAGE with a default before FROM.

These run without Docker, so they execute in CI as part of npm test.

Files changed

File	Change
`src/spec-configuration/containerFeaturesConfiguration.ts`	Declare `ARG _DEV_CONTAINERS_BASE_IMAGE=scratch` before `FROM`
`src/spec-node/containerFeatures.ts`	ARG default `placeholder` → `scratch` in both build-option paths
`scripts/updateUID.Dockerfile`	`ARG BASE_IMAGE` → `ARG BASE_IMAGE=placeholder`
`src/test/container-features/generateFeaturesConfig.test.ts`	Add `InvalidDefaultArgInFrom` regression tests

Notes / follow-ups

The getImageBuildOptions no-features path is covered indirectly (its ARG default was corrected), but its Dockerfile string is built inline and isn't directly unit-tested. A small follow-up could extract that string into an exported helper to unit-test it the same way.
.devcontainer/devcontainer-lock.json shows a trailing-newline-only change; consider reverting that if it's unintentional.

Abdurrahmaan Iqbal (abdurriq) · 2026-07-28T13:28:39Z

+});
+
+// returns the names of any ARGs used in a `FROM` that lack a default declared before them.
+function findFromArgsWithoutDefault(dockerfile: string): string[] {


Do we not also want to use this for other features?

Hi Abdurrahmaan Iqbal (@abdurriq)

Thank you for pointing this out. It's done now. Kindly let me know in case of any concern.

Vatsal Gupta (gvatsal60) · 2026-08-06T13:56:50Z

Hi Kaniska (@Kaniska244),
Would it be possible to merge this PR when you get a chance?

The current behaviour is breaking existing workflows.

…ependency of github-cli feature.

Copilot

Pull request overview

This PR addresses Docker/BuildKit’s InvalidDefaultArgInFrom warning by ensuring Dockerfiles (generated by the CLI and scripts/updateUID.Dockerfile) declare a default value for any ARG referenced by a FROM, using syntax the linter recognizes.

Changes:

Set _DEV_CONTAINERS_BASE_IMAGE default to scratch in generated Dockerfile prefixes and in the feature base Dockerfile template.
Set BASE_IMAGE=placeholder in scripts/updateUID.Dockerfile to avoid FROM $BASE_IMAGE default warnings.
Add unit tests to regression-check generated Dockerfiles for ARG defaults before FROM, plus a helper to detect violations.

Show a summary per file

File	Description
`src/test/testUtils.ts`	Adds a helper to detect `FROM`-referenced args lacking defaults (used by new tests).
`src/test/container-features/generateFeaturesConfig.test.ts`	Adds regression tests ensuring generated Dockerfiles avoid `InvalidDefaultArgInFrom`.
`src/spec-node/containerFeatures.ts`	Changes generated Dockerfile prefix default `_DEV_CONTAINERS_BASE_IMAGE` from `placeholder` to `scratch`.
`src/spec-configuration/containerFeaturesConfiguration.ts`	Adds `ARG _DEV_CONTAINERS_BASE_IMAGE=scratch` before template `FROM` stages.
`scripts/updateUID.Dockerfile`	Adds default `BASE_IMAGE=placeholder` to satisfy Dockerfile parsing/linting.
`.devcontainer/devcontainer-lock.json`	Trailing newline / formatting-only change.

Review details

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Files reviewed: 5/6 changed files
Comments generated: 1
Review effort level: Lite

+export function findFromArgsWithoutDefault(dockerfile: string): string[] {
+    const argsWithDefault = new Set<string>();
+    const offenders: string[] = [];
+
+    for (const rawLine of dockerfile.split('\n')) {
+        const line = rawLine.trim();
+        const argWithDefault = /^ARG\s+([A-Za-z0-9_]+)\s*=\s*\S+/.exec(line);
+        if (argWithDefault) {
+            argsWithDefault.add(argWithDefault[1]);
+            continue;
+        }
+        if (/^ARG\s+[A-Za-z0-9_]+\s*$/.test(line)) {
+            continue;
+        }
+        const fromMatch = /^FROM\s+\$\{?([A-Za-z0-9_]+)/.exec(line);
+        if (fromMatch && !argsWithDefault.has(fromMatch[1])) {
+            offenders.push(fromMatch[1]);
+        }
+    }
+    return offenders;
+}


Kaniska (Kaniska244) added 2 commits July 28, 2026 09:07

fix: set default base image in Dockerfiles and improve syntax handling

6a9bce6

Retrigger test

720f31d

Kaniska (Kaniska244) marked this pull request as ready for review July 28, 2026 09:24

Kaniska (Kaniska244) requested a review from a team as a code owner July 28, 2026 09:24

Abdurrahmaan Iqbal (abdurriq) reviewed Jul 28, 2026

View reviewed changes

Addressing review comments.

dacfdf5

Kaniska (Kaniska244) requested a review from Abdurrahmaan Iqbal (abdurriq) August 6, 2026 15:59

Kaniska (Kaniska244) added 4 commits August 7, 2026 06:26

Retrigger tests

6866d07

Fixing failing tests due to change in terraform feature to add hard d…

4fb229b

…ependency of github-cli feature.

Removing unrelated test fixes

d93cb60

Merge branch 'main' into invalid-arg-fix

c37262f

Kaniska (Kaniska244) requested a lite review from Copilot August 12, 2026 11:19

Copilot started reviewing on behalf of Kaniska (Kaniska244) August 12, 2026 11:20 View session

Copilot AI reviewed Aug 12, 2026

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fix: set default base image in Dockerfiles and improve syntax handling - #1271

fix: set default base image in Dockerfiles and improve syntax handling#1271
Kaniska (Kaniska244) wants to merge 7 commits into
devcontainers:mainfrom
Kaniska244:invalid-arg-fix

Kaniska (Kaniska244) commented Jul 28, 2026 •

edited

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Abdurrahmaan Iqbal (abdurriq) Jul 28, 2026

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Kaniska (Kaniska244) commented Jul 28, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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What this PR does

Root cause

The fix

Behavior is unchanged at build time

Tests

Files changed

Notes / follow-ups

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Abdurrahmaan Iqbal (abdurriq) Jul 28, 2026

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Kaniska (Kaniska244) Aug 6, 2026

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Vatsal Gupta (gvatsal60) commented Aug 6, 2026

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Kaniska (Kaniska244) commented Jul 28, 2026 •

edited

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