fix: expand tilde in --env-file paths to user home directory

[tensorworkerr]

When using --env-file=/~.env, the tilde was not expanded to the user's home directory. Instead, it was treated as a literal character and resolved relative to the current working directory, resulting in errors like "couldn't find env file: /current/dir/~/.env".

This adds an ExpandUser function that expands ~ to the home directory before converting relative paths to absolute paths.
Fixes #13508

What I did
-Added ExpandUser() function in internal/paths/paths.go that expands a leading ~ to the user's home directory using os.UserHomeDir()
-Updated the env file path processing in cmd/compose/compose.go to call ExpandUser() before checking if paths are absolute
-Added unit tests for the new function covering various edge cases (empty string, ~ only, ~/.env, ~/subdir/.env, absolute paths, relative paths, tilde in middle of path, ~otheruser/... syntax)

Related issue
Fixes #13508

Contribution by Gittensor, see my contribution statistics at https://gittensor.io/miners/details?githubId=178221229

[ndeloof]

you can use paths.ExpandUser from compose-go

[tensorworkerr]

Updated as you suggested.

[ndeloof]

A better place to fix this is GetEnvFromFile in compose-go: relativePathsResolver already manages this for usage within a compose file as file attributes, would make sense to have this logic all in one place

[thaJeztah]

@ndeloof I don't think this was a bug, and actually the correct behavior; expansion is handled by the shell, and does not happen if the value is prefixed by a =; you'll see the same for any tool, e.g. trying with git;

git --git-dir=~/go/src/github.com/docker/compose/.git describe
fatal: not a git repository: '~/go/src/github.com/docker/compose/.git'

git --git-dir ~/go/src/github.com/docker/compose/.git describe
v2.26.1-891-gb760afaf9

Bit concerned that this could be a sliding slope.

[ndeloof]

@thaJeztah yes I know shell is in charge for this, but as long as we support both space and equal flag values, need to get an homogeneous behavior. compose.yaml already offers this feature

[thaJeztah]

Yeah, the issue here would be that it's not homogeneous across the CLI; other commands will not expand these, so now docker <some command> --some-flag=~/foo won't work, but docker compose --some-flag=~/foo will.