fix: docker backend should not close 'engine.Tail' result

[alexmt]

Closes #1615

The error described in #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:

woodpecker/pipeline/backend/docker/docker.go

Line 229 in 2171212

_ = rc.Close()

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:

woodpecker/pipeline/pipeline.go

Lines 231 to 237 in 2171212

	defer wg.Done()
	logger := r.MakeLogger()
	if err := r.logger.Log(step, multipart.New(rc)); err != nil {
	logger.Error().Err(err).Msg("process logging failed")
	}
	_ = rc.Close()

Example of woodpecker exec output using pipeline from #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

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.05 ⚠️

Comparison is base (a2f226f) 40.64% compared to head (c20c442) 40.59%.

❗ Current head c20c442 differs from pull request most recent head 08e6654. Consider uploading reports for the commit 08e6654 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1616      +/-   ##
==========================================
- Coverage   40.64%   40.59%   -0.05%     
==========================================
  Files         169      169              
  Lines       11083    11084       +1     
==========================================
- Hits         4505     4500       -5     
- Misses       6287     6293       +6     
  Partials      291      291              

Impacted Files	Coverage Δ
pipeline/backend/docker/docker.go	0.00% <ø> (ø)
server/model/task.go	0.00% <ø> (ø)
server/queue/persistent.go	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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[6543]

backport -> #1620