Multi-variable ARG declared via line continuation loses its second variable

Summary

When a Dockerfile declares two ARGs using a single instruction with a backslash line continuation (valid, real Docker/BuildKit syntax):

ARG VERSION_UNUSED=x \
    VERSION_BASE=latest
FROM alpine:${VERSION_BASE}

docker build resolves ${VERSION_BASE} to latest correctly. But @devcontainers/cli's own lightweight Dockerfile scanner (used to pre-fetch base-image details before invoking the real build) never sees VERSION_BASE as a declared ARG at all, so ${VERSION_BASE} resolves to an empty string. The image reference becomes alpine: (no tag), which is rejected by the Docker daemon as invalid reference format. In our case this made Reopen in Container fail outright, before the actual build (which would have succeeded) ever ran.

Environment

• Dev Containers extension: 0.466.0
• VS Code: 1.134.0
• @devcontainers/cli (bundled): 0.88.0
• Docker Desktop: 4.87.0 (Engine 29.7.2)
• macOS (arm64)

Minimal reproduction

.devcontainer/Dockerfile:

ARG VERSION_UNUSED=x \
    VERSION_BASE=latest
FROM alpine:${VERSION_BASE}

.devcontainer/devcontainer.json:

{
  "name": "arg-continuation-repro",
  "dockerComposeFile": "../docker-compose.yml",
  "service": "web",
  "workspaceFolder": "/workspace"
}

docker-compose.yml:

services:
  web:
    build:
      context: .
      dockerfile: .devcontainer/Dockerfile
      args:
        VERSION_BASE: "3.20"
    command: ["sleep", "infinity"]

Steps:

1. Save the three files above in an otherwise empty folder.
2. Open the folder in VS Code and run Dev Containers: Reopen in Container.

Expected result

The container builds using alpine:3.20 (from the compose build.args override) or alpine:latest (the Dockerfile default) — either way, a valid tag.

Actual result (from our real project, same root cause, different image name)

[...] Start: Run: docker inspect --type image futureys/gatsby:
[...] Stop (11 ms): Run: docker inspect --type image futureys/gatsby:
[...] Error fetching image details: No manifest found for docker.io/futureys/gatsby:.
[...] Start: Run: docker pull futureys/gatsby:
[...] invalid reference format
[...] Stop (137 ms): Run: docker pull futureys/gatsby:
[...] Retrying (Attempt 0) with error
      'Command failed: docker pull futureys/gatsby:
'
... (retries 1-4, same error) ...
[...] Command failed: docker inspect --type image futureys/gatsby:
[...] []
Error response from daemon: invalid reference format

[...] Command failed: docker pull futureys/gatsby:
Error: Command failed: docker pull futureys/gatsby:

Reopen in Container fails with this error before the real docker compose build is ever invoked, even though docker build on the same Dockerfile (outside of Dev Containers) succeeds and resolves the ARG correctly.

Root cause

The bundled devContainersSpecCLI.js parses ARG/ENV/USER declarations with this regex (used to compute the default value of an ARG referenced in FROM ...:${ARG} before the real build runs, e.g. to fetch base-image metadata):

/^\s*(?<instruction>ARG|ENV|USER)\s+(?<name>[^\s=]+)([ =]+("(?<value1>\S+)"|(?<value2>\S+)))?/gmi

Because of the m (multiline) flag, ^ anchors to the start of each physical line. For:

ARG VERSION_UNUSED=x \
    VERSION_BASE=latest

only the first line matches (name=VERSION_UNUSED). The second physical line ( VERSION_BASE=latest) doesn't start with ARG/ENV/USER, so it's invisible to this scanner — even though real Docker/BuildKit joins the two lines via the trailing \ into one ARG instruction that declares both variables (verified: docker build on the same Dockerfile resolves ${VERSION_BASE} to latest/the passed --build-arg correctly).

Downstream, the variable-resolution walk (roughly Iv → wg → LV → Cv in the minified bundle) looks for an instruction whose name matches the referenced variable. Since no such instruction was ever recorded for the second, continuation-declared name, the lookup falls through and resolves to undefined → "", which gets substituted into the FROM line, producing image: with no tag.

Workaround

Declare each ARG on its own line instead of combining them into one instruction via \ continuation:

ARG VERSION_UNUSED=x
ARG VERSION_BASE=latest

This is recognized correctly by both real Docker/BuildKit and the extension's scanner.

Suggested fix

The Dockerfile-instruction regex (and/or the code that walks continuation lines) should join \-continued lines before matching, the same way BuildKit's own parser does — or, at minimum, should recognize additional name[=value] pairs appearing on continuation lines of an ARG/ENV instruction.