Lowercase container hostnames

[mvsusp]

Description of changes:

• Docker hostnames are case sensitive. OpenSSH lowercases the hostnames before the
  handshake, given that UNIX hostnames should be case insensitive. This small change
  allows to run SSH inside local mode.

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have updated the changelog with a description of my changes (if appropriate)
• I have updated any necessary documentation (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[laurenyu]

changelog entry?

[codecov-io]

Codecov Report

Merging #559 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #559   +/-   ##
=======================================
  Coverage   92.79%   92.79%           
=======================================
  Files          71       71           
  Lines        5372     5372           
=======================================
  Hits         4985     4985           
  Misses        387      387

Impacted Files	Coverage Δ
src/sagemaker/local/image.py	88.53% <100%> (ø)	⬆️

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[ChoiByungWook]

LGTM