update Dockerfile to install pydot

[Nxtra]

Description of changes

Current behaviour
Unable to generate the graph .dot file that visualizes the dependency graph when using cfn-lint in docker.

Future behaviour
Being able to generate the dependency graph using the docker container of cfn-lint.
Now the file can be generated:

Updates

• install pydot in the docker container.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Merging #1490 into master will not change coverage.
The diff coverage is 91.93%.

@@           Coverage Diff           @@
##           master    #1490   +/-   ##
=======================================
  Coverage   87.54%   87.54%           
=======================================
  Files         156      156           
  Lines        8782     8782           
  Branches     2101     2101           
=======================================
  Hits         7688     7688           
  Misses        657      657           
  Partials      437      437           

Impacted Files	Coverage Δ
src/cfnlint/helpers.py	78.60% <87.80%> (ø)
src/cfnlint/maintenance.py	98.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3b706b2...8c8e72d. Read the comment docs.