Improve Docker daemon error for flowctl runtime

[RitwijParmar]

Summary

• detect Docker daemon/connectivity errors even when the docker CLI exits successfully with the real failure on stderr
• return a clear flowctl-oriented error before empty docker inspect output reaches JSON parsing
• apply the fix to both runtime and runtime-next container paths with focused unit coverage

Fixes #2969.

Tests

• cargo fmt --check
• cargo test -p runtime detects_docker_daemon_unavailable_from_stderr --lib
• cargo test -p runtime ignores_unrelated_docker_stderr --lib
• cargo test -p runtime-next detects_docker_daemon_unavailable_from_stderr --lib
• cargo test -p runtime-next ignores_unrelated_docker_stderr --lib
• git diff --check