Description of the false positive
This was originally reported for zizmorcore/zizmor#1705, please see the ticket for more context and an SSCCE. It's been explained that zizmor is showing that vulnerability report as a result of this entry in CodeQL:

Code samples or links to source code
The full, step-by-step analysis was documented here: zizmorcore/zizmor#1705 (comment), but for the sake of clarity:

1. docker/build-push-action passes context as a string item in an array:
   • Creation of the array:
     • https://github.com/docker/build-push-action/blob/d08e5c354a6adb9ed34480a06d141179aa583294/src/context.ts#L84-L94
     • https://github.com/docker/build-push-action/blob/master/src/main.ts#L95
   • Passing it further:
     https://github.com/docker/build-push-action/blob/master/src/main.ts#L103-L109
2. That last call to Exec.getExecOutput(...) leads to docker/actions-toolkit, which still processes all args as an array:
   https://github.com/docker/actions-toolkit/blob/main/src/exec.ts#L27-L30
3. The implementation of getExecOputpt(...) calls exec(...) in the same class:
   • https://github.com/actions/toolkit/blob/main/packages/exec/src/exec.ts#L79
   • https://github.com/actions/toolkit/blob/main/packages/exec/src/exec.ts#L17-L31
4. The exec(...) functions processes the args array, but only to extract the first element of it (the command - build or buildx depending on the earlier setup), and passes the args array to ToolRunner class, in the same library.
5. The implementation of ToolRunner only processes the args via _getSpawnArgs function, which still returns them as array and then uses Node.js built-in child_process.spawn() (see here: https://github.com/actions/toolkit/blob/main/packages/exec/src/toolrunner.ts#L437-L441) to invoke docker.

The args are being passed down as an array through all these layers, and finally end up in a Node.js built-in. So, unless the're a vulnerability in Node.js child_process.spawn, passing the context as . --buildArg FOO=foo will not cause the FOO build argument to be passed to docker. Instead the whole thing will be treated as context path and cause docker build command to fail.