How can I build Git submodules in Docker when the .git directory is inaccessible?

Below is one way to think about alternatives:

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Option 1: Try to “self‑contain” the .git directory

In older versions of git submodules the checkout would include a full .git folder. Today git instead puts a pointer to the parent’s .git directories by default. There’s a command called “git submodule absorbgitdirs” which can sometimes be used to move the git metadata into the submodule’s work tree, but it comes with caveats (for example, it makes some operations more difficult and might break standard workflows). Therefore, it’s not always a safe bet for production builds.

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Option 2: Use separate clones instead of submodules

This approach sidesteps the git directory location issue because each cloned repository would have its own .git folder in its project root, making git describe available inside the Docker instance as before. If you already have scripts that support a stand-alone project, this is a pragmatic solution and minimizes the need to modify the submodule build scripts.

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Option 3: Pass git metadata from projU

You can have projU extract the required git commit/describe info from each submodule and inject this information into the containers as environment variables (or even into a file mounted into the container). Then the submodule build scripts could use the passed-in info, bypassing the need to locate the .git directory. This can be implemented with a wrapper script in projU that:

• Queries git describe (or similar) for each submodule.
• Passes the resulting string into the docker run command as an environment variable, e.g., -e PROJ_A_VERSION="v1.2.3".
• Updates the build procedure in projA (and projB) to read that environment variable if available.

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Option 4: Mount the umbrella's root in Docker

Another approach is to have projU’s build script mount the entire projU into the docker container instead of only the individual submodule’s root. In this setup the submodule’s build script could potentially traverse to the parent directory to find the full .git context. However, this introduces extra complexity—every script must be aware of the adjusted path and there is a risk of interfering with path assumptions inside the submodule scripts.

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Alternative: Hybrid/Wrapper Solution

A more flexible solution might be to create a wrapper build layer in projU that detects whether the build is being done in a “stand-alone” context or as a submodule, and then adapts accordingly. For example:

• Detection: Check whether .git exists in the current project root (or whether the git info appears to be a pointer file).
• Action: If the repository is a submodule (or the .git folder is missing), then either (a) use the metadata passed from projU (via envvars or a configuration file), or (b) mount the full projU repository.
• Fallback: If it’s a full clone, use the standard git describe.

This wrapper logic can be centralized so that you do not need to maintain multiple build scripts for each project.

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Summary and Recommendation

All options have pros and cons:

• Option 1 is appealing from a “pure” repo standpoint but can be brittle with modern git.
• Option 4 might work but requires significant changes, including altering assumptions inside the existing build scripts.
• Option 2 (using separate clones) is straightforward and minimizes changes to the existing scripts.
• Option 3 offers a middle ground: you preserve the repo structure (using submodules), yet you avoid needing local .git directories inside your containers by injecting the necessary information at build time.

If you want to avoid redundant scripts and keep the existing build logic in each submodule, Option 2 or Option 3 tend to be the most pragmatic. Many teams choose Option 3 because it lets you keep the umbrella structure (with submodules) and perform the “git information extraction” once, then pass it into the builds.

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Choose the approach that best fits your workflow and the level of coupling you want to have between your umbrella project and the individual components.