BF: docker adapter: Don't couple 'run -i' with stdin.isatty

[kyleam]

24af27f (BF: docker adapter: Drop -it flags when stdin is not a tty,
2019-11-11) dropped both --interactive and --tty from the 'docker run'
call if stdin.isatty() reports false, avoiding an "the input device is
not a TTY" failure in this case.

However, dropping --interactive shouldn't be necessary, and doing so
prevents the command from receiving stdin.

---

I noticed this issue when looking into gh-123.

Before this PR:

$ echo foo | md5sum
d3b07384d113edec49eaa6238ad5ff00  -
$ echo foo | datalad containers-run -n alp md5sum
[INFO] Making sure inputs are available (this may take some time)
[INFO] == Command start (output follows) =====
d41d8cd98f00b204e9800998ecf8427e  -
[INFO] == Command exit (modification check follows) =====
[INFO] Total: starting
[INFO]
[INFO] Total: processed result for /tmp/po-TYjIvuy
[INFO] Total: done
action summary:
  get (notneeded: 1)
  save (notneeded: 1)

After:

$ echo foo | datalad containers-run -n alp md5sum
[INFO] Making sure inputs are available (this may take some time)
[INFO] == Command start (output follows) =====
d3b07384d113edec49eaa6238ad5ff00  -
[INFO] == Command exit (modification check follows) =====
[INFO] Total: starting
[INFO]
[INFO] Total: processed result for /tmp/po-TYjIvuy
[INFO] Total: done
action summary:
  get (notneeded: 1)
  save (notneeded: 1)

[codecov]

Codecov Report

Merging #142 (0624181) into master (d958d80) will increase coverage by 0.04%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #142      +/-   ##
==========================================
+ Coverage   86.50%   86.54%   +0.04%     
==========================================
  Files          17       17              
  Lines         926      929       +3     
==========================================
+ Hits          801      804       +3     
  Misses        125      125              

Impacted Files	Coverage Δ
datalad_container/adapters/docker.py	35.95% <0.00%> (ø)
datalad_container/adapters/tests/test_docker.py	93.54% <100.00%> (+0.32%)	⬆️

---

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[kyleam]

I'll plan to wait another day or two for objections and then merge.