fix: docker backend should not close 'engine.Tail' result (#1616)

Closes https://github.com/woodpecker-ci/woodpecker/issues/1615

The error described in
https://github.com/woodpecker-ci/woodpecker/issues/1615 is happening
because `Tail` method of the docker backend closes the instance of
`io.ReadCloser` it returns in `defer` function. As a result anything
that try to read data returned by `Tail` method eventually will attempt
to read from closes reader and get an error:


2171212c5a/pipeline/backend/docker/docker.go (L229)

The fix is just don't close returned reader and let the consumer of
`Tail` method do it. Good thing is that `Tail` is used only in one place
and reader is correctly closed:


2171212c5a/pipeline/pipeline.go (L231-L237)

Example of `woodpecker exec` output using pipeline from
https://github.com/woodpecker-ci/woodpecker/issues/1615 with the fix:

```
woodpecker exec .woodpecker.yaml
[step1:L0:0s] + echo step1
[step1:L1:0s] step1
[step2:L0:0s] + echo step2
[step2:L1:0s] step2
```

Signed-off-by: Alexander Matyushentsev <AMatyushentsev@gmail.com>
Co-authored-by: Lauris BH <lauris@nix.lv>

pipeline/backend/docker/docker.go

@@ -226,7 +226,6 @@ func (e *docker) Tail(ctx context.Context, step *backend.Step) (io.ReadCloser, e
 		_, _ = stdcopy.StdCopy(wc, wc, logs)
 		_ = logs.Close()
 		_ = wc.Close()
-		_ = rc.Close()
 	}()
 	return rc, nil
 }