This is a minor fix that tries to replace `"workdir"` with const in `service create`.
Since `"workdir"` is the only string not defined as const in `func addServiceFlags()`,
I think it makes sense to replace `"workdir"` with a const to be consistent.
The flag `"workdir"` in `service update` has also been replaced.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
Upstream-commit: 55d05fc0556dc7d3ff3c65b8d739e48a1f79afab
Component: engine