fix(docker): fix streaming of combined stdout/stderr

[tcolgate]

This PR fixes #2261. Multireader concactinates, not combines, the two
streams, this meant that we blocked on stdin if stderr was not
completely buffered.

In addition, this implementation ensures that we call Wait() on the
command, and any error is return by Close(). Godoc for
exec.Cmd.Std(out|err)Pipe, confirms that we do not need to close those
pipes.

This PR also ensures that we do not leak go routines in the even that
cmd.Start() fails.

[codecov]

Codecov Report

Merging #2368 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2368   +/-   ##
=======================================
  Coverage   13.28%   13.28%           
=======================================
  Files          72       72           
  Lines       24653    24653           
=======================================
  Hits         3274     3274           
  Misses      20966    20966           
  Partials      413      413

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[tcolgate]

tested with:

kind: Workflow
metadata:
  name:  wftestthing.3
  namespace: argo
spec:
  entrypoint: build
  templates:
  - container:
      args:
      - "for i in $(seq 100000); do date > /dev/stderr;  done "
      command:
      - sh
      - -c
      image: alpine:latest
    name: build```

[alexec]

Interesting fix. Thank you.