Support arbitrary Dockerfile in hive cli

[ky-hiverge]

What would you like to be added:

We generate sandbox images with two approaches, one is from the user repo Dockerfile, another is to generate a Dockerfile ourselves if user doesn't provide one.

However, because of this configuration:

hive-cli/libs/main.py

Line 12 in 2934515

REPO_DIR = "/app/repo" # Directory where the repository is mounted

We have to make sure the sandbox image has the /app folder, which is not very straightforward to users. We should optimize this.

cc @brp-hiverge

Why is this needed:

Easy setup from user side.

[ky-hiverge]

Should be solved by #21 as well.