Regression: Dockerfile build context defaults to the Dockerfile's own directory again (PR #5271 reverted the fix from #5231)

To Reproduce

1. Have a repo where a Dockerfile lives in a subdirectory and references a file that lives elsewhere in the repo, e.g.:

   .
   ├── shared/
   │   └── config.json
   └── docker/
       └── api/
           └── Dockerfile
   

   with docker/api/Dockerfile containing:

   FROM alpine
   COPY shared/config.json /app/config.json
   

2. In Dokploy, create a Dockerfile-type application and set the Dockerfile path to docker/api/Dockerfile.
3. Leave the Docker Context Path field empty (its placeholder implies the default is ., i.e. the repo root).
4. Deploy.
5. The build fails because shared/config.json isn't present in the build context — Dokploy used docker/api/ as the context instead of the repo root:

   #5 [2/2] COPY shared/config.json /app/config.json
   #5 ERROR: failed to calculate checksum of ref ...: "/shared/config.json": not found
   

For comparison, an equivalent Compose-type app with compose.yaml declaring context: . and dockerfile: docker/api/Dockerfile builds the same Dockerfile correctly, since it actually uses the repo root as context.

Current vs. Expected behavior

Expected: with Docker Context Path left empty, the build context defaults to the application's code root ., matching the field's own placeholder and matching how docker compose build resolves the same Dockerfile.

Actual: the build context defaults to the Dockerfile's own directory instead, so any file reference outside that directory 404s during the build.

Provide environment information

Operating System:
  OS: Ubuntu (self-hosted VPS)
  Arch: amd64
Dokploy version: 0.30.6
VPS Provider: (self-hosted / on-prem)
What applications/services are you trying to deploy?
  Dockerfile-type applications in a monorepo, where the Dockerfile lives in a
  subdirectory (e.g. docker/<service>/Dockerfile) and COPYs files from the repo
  root, matching the context: . setting used by the equivalent compose.yaml.

Which area(s) are affected? (Select all that apply)

Docker, Application

Are you deploying the applications where Dokploy is installed or on a remote server?

Same server where Dokploy is installed

Additional context

No response

Will you send a PR to fix it?

Maybe, need help

Start by comparing Dockerfile-type application context resolution with the compose.yaml behavior described in the issue, focusing on how an empty Docker Context Path is interpreted when the Dockerfile is in a subdirectory. Reproduce the example with docker/api/Dockerfile and shared/config.json, then verify that the build uses the repository root as context and completes successfully.