fix(execd): Add fallback from bash to sh for Alpine-based images

[Jah-yee]

Summary

This PR addresses issue #405 by adding a fallback from bash to sh for Alpine-based Docker images.

Problem

In a lot of Docker images like Alpine-based images, they only contain sh. They do not contain bash by default, making it impossible to run any command using the execd.

Solution

Added a getShell() helper function that:

1. First tries to find bash using exec.LookPath
2. Falls back to sh if bash is not available

This change has been applied to both locations where shell commands are executed (lines ~96 and ~231).

Testing

The fix is backward compatible - on systems with bash (like Ubuntu, Debian), it will continue to use bash. On Alpine-based images, it will automatically fall back to sh.

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[Pangjiping]

Thanks for fixes. Please sign CLA and I will review and merge those changes as soon as possible.

[Pangjiping]

Perhaps the bootstrap.sh script also needs to be compatible with sh-only environments.

ref: https://github.com/alibaba/OpenSandbox/blob/main/components/execd/bootstrap.sh

set -x
if [ "$CMD" != "" ]; then
exec bash -c "$CMD"
fi

if [ $# -eq 0 ]; then
exec bash
fi