SYN-1388: Add devcontainer.json declarative env definition to dev-sandbox

[JoshuaPurtell]

Resolves https://linear.app/synth-ai/issue/SYN-1388/add-devcontainerjson-declarative-env-definition-to-dev-sandbox

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 3 additional findings in Devin Review.

[devin-ai-integration]

🔴 Dockerfile path resolves to non-existent .devcontainer/Dockerfile

The devcontainer.json specifies "dockerfile": "Dockerfile" at line 8. Per the devcontainer spec, the dockerfile path is resolved relative to the devcontainer.json file, which means it resolves to .devcontainer/Dockerfile. However, no such file exists in the .devcontainer/ directory — only devcontainer.json is present.

Root Cause and Fix

The intent is to reuse the Dockerfile from the sibling synth-bazel/dev-sandbox repo (as stated in the README at README.md:245). The build.context is correctly set to "../../synth-bazel/dev-sandbox" (resolving to ../synth-bazel/dev-sandbox/ from the repo root), but the dockerfile field needs to point to the Dockerfile within that context directory.

The dockerfile should be "../../synth-bazel/dev-sandbox/Dockerfile" to correctly reference the Dockerfile in the sibling repo, relative to the devcontainer.json location.

Impact: Running devcontainer up --workspace-folder . (as documented in README.md:250) will fail because Docker cannot find the Dockerfile, making the entire dev container setup non-functional.

Suggested change

	"dockerfile": "Dockerfile",
	"dockerfile": "../../synth-bazel/dev-sandbox/Dockerfile",

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[greptile-apps]

Greptile Summary

Added declarative devcontainer configuration that standardizes dev-sandbox setup across Colima and Daytona environments. The .devcontainer/devcontainer.json references an external Dockerfile from the sibling synth-bazel repo, sets up the workspace at /workspace/synth-ai, and runs the enhanced setup.sh script.

• New .devcontainer/devcontainer.json provides declarative environment spec reusing synth-bazel/dev-sandbox Dockerfile
• Updated .sandbox/setup.sh to honor WORKSPACE_DIR environment variable with proper quoting
• Added comprehensive README documentation covering Colima bootstrapping, Daytona integration, and multi-repo extension patterns

One note: the devcontainer build relies on ../../synth-bazel/dev-sandbox/Dockerfile existing at that relative path, which requires the sibling repo to be checked out alongside this one.

Confidence Score: 4/5

• Safe to merge with verification that external Dockerfile dependency exists
• Changes are well-structured and properly implement devcontainer spec with improved shell script using proper variable quoting and defaults. Documentation is thorough. Only concern is the external dependency on ../../synth-bazel/dev-sandbox/Dockerfile which requires the sibling repo to be present.
• Verify .devcontainer/devcontainer.json build context path points to existing Dockerfile

Important Files Changed

Filename	Overview
.devcontainer/devcontainer.json	New devcontainer config added with external Dockerfile reference; requires sibling synth-bazel repo at ../../synth-bazel/dev-sandbox
.sandbox/setup.sh	Improved to use WORKSPACE_DIR env var with proper quoting and default value
README.md	Added comprehensive documentation for dev-sandbox setup, Colima, Daytona, and multi-repo usage

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[devcontainer up] --> B[Load devcontainer.json]
    B --> C[Build from ../../synth-bazel/dev-sandbox/Dockerfile]
    C --> D[Mount workspace at /workspace/synth-ai]
    D --> E[Set WORKSPACE_DIR env var]
    E --> F[Run postCreateCommand]
    F --> G[Execute ./.sandbox/setup.sh]
    G --> H{Check for uv command}
    H -->|Found| I[cd $WORKSPACE_DIR]
    I --> J[git config safe.directory]
    J --> K[uv sync --frozen]
    K --> L[maturin develop --release --uv]
    L --> M[Dev sandbox ready]
    H -->|Not found| N[Exit with error]

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_{Last reviewed commit: 5349378}

[greptile-apps]

_{3 files reviewed, 1 comment}

_{Edit Code Review Agent Settings | Greptile}

[greptile-apps]

Verify synth-bazel/dev-sandbox/Dockerfile exists at this relative path - devcontainer will fail to build if sibling repo is missing or in different location