fix: Improve context path handling custom Dockerfiles

[verils]

I fixed an issue that occurred in v1.5.0.

When I built the AutoGPT project using podman-compose, I encountered the following problem:

bob@krusty-krab:~/Projects/git/AutoGPT/autogpt_platform$ podman compose --verbose build 
>>>> Executing external compose provider "/usr/bin/podman-compose". Please see podman-compose(1) for how to disable this message. <<<<

INFO:podman_compose:['podman', '--version', '']
INFO:podman_compose:using podman version: 5.7.0
INFO:podman_compose:podman build -f ../../autogpt_platform/backend/Dockerfile -t autogpt_platform_migrate --target migrate ../
INFO:podman_compose:podman build -f ../../autogpt_platform/backend/Dockerfile -t autogpt_platform_database_manager --target server ../
INFO:podman_compose:podman build -f ../../autogpt_platform/frontend/Dockerfile -t autogpt_platform_frontend --target prod --build-arg NEXT_PUBLIC_PW_TEST=false ../
INFO:podman_compose:podman build -f ../../autogpt_platform/backend/Dockerfile -t autogpt_platform_rest_server --target server ../
INFO:podman_compose:podman build -f ../../autogpt_platform/backend/Dockerfile -t autogpt_platform_websocket_server --target server ../
INFO:podman_compose:podman build -f ../../autogpt_platform/backend/Dockerfile -t autogpt_platform_notification_server --target server ../
INFO:podman_compose:podman build -f ../../autogpt_platform/backend/Dockerfile -t autogpt_platform_executor --target server ../
INFO:podman_compose:podman build -f ../../autogpt_platform/backend/Dockerfile -t autogpt_platform_scheduler_server --target server ../
Error: the specified Containerfile or Dockerfile does not exist, ../../autogpt_platform/backend/Dockerfile: no such file or directory
INFO:podman_compose:exit code: 125
Error: the specified Containerfile or Dockerfile does not exist, ../../autogpt_platform/backend/Dockerfile: no such file or directory
INFO:podman_compose:exit code: 125
Error: the specified Containerfile or Dockerfile does not exist, ../../autogpt_platform/backend/Dockerfile: no such file or directory
INFO:podman_compose:exit code: 125
Error: the specified Containerfile or Dockerfile does not exist, ../../autogpt_platform/backend/Dockerfile: no such file or directory
INFO:podman_compose:exit code: 125
Error: the specified Containerfile or Dockerfile does not exist, ../../autogpt_platform/backend/Dockerfile: no such file or directory
INFO:podman_compose:exit code: 125
Error: the specified Containerfile or Dockerfile does not exist, ../../autogpt_platform/backend/Dockerfile: no such file or directory
INFO:podman_compose:exit code: 125
Error: the specified Containerfile or Dockerfile does not exist, ../../autogpt_platform/backend/Dockerfile: no such file or directory
INFO:podman_compose:exit code: 125
Error: the specified Containerfile or Dockerfile does not exist, ../../autogpt_platform/frontend/Dockerfile: no such file or directory
INFO:podman_compose:exit code: 125
Error: executing /usr/bin/podman-compose --verbose build: exit status 125

Here's a fragment of the docker-compose.yml file in AutoGPT/autogpt_platform/:

services:
  migrate:
    build:
      context: ../
      dockerfile: autogpt_platform/backend/Dockerfile

I wrote some tests for container_to_build_args() function in podman_compose.py and found that it always concatenated the context path twice as the prefix of dockerfile path. For example, when I used context: subdir/, container_to_build_args() would generate build args like ... -f subdir/subdir/<dockerfile> ...

There are two different logical dockerfile paths when handling the -f, --file build argument with a local context (i.e., when the build context comes from the local filesystem):

1. when checking if the dockerfile exists: the file path should be relative to the cwd where the podman-compose command is run.
2. when using the -f build arg: the file path should be relative to the context root.

I fixed this issue and also added test cases to cover these scenarios.