Add Docker support

[maliasadi]

Context:
Add Docker support to setup & test PL-Lightning.

Description of the Change:

• Add Dockerfile
• Update README
• Update cmake/process_options.cmake
• Update setup.py

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit .github/CHANGELOG.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
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[codecov]

Codecov Report

Merging #234 (db35c2e) into master (2cb2727) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #234   +/-   ##
=======================================
  Coverage   99.71%   99.71%           
=======================================
  Files           4        4           
  Lines         354      354           
=======================================
  Hits          353      353           
  Misses          1        1           

Impacted Files	Coverage Δ
pennylane_lightning/lightning_qubit.py	99.62% <ø> (ø)
pennylane_lightning/_version.py	100.00% <100.00%> (ø)

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[github-actions]

Test Report (C++) on Ubuntu

       1 files  ±0         1 suites  ±0   0s ⏱️ ±0s
   555 tests ±0     555 ✔️ ±0  0 💤 ±0  0 ❌ ±0 
2 289 runs  ±0  2 289 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit db35c2e. ± Comparison against base commit 2cb2727.

♻️ This comment has been updated with latest results.

[mlxd]

Hey @maliasadi thanks for putting this together.
Just curious, how does one expect to use the Docker version of lightning?

Will this be strictly used for building wheels, building and running the test-suite, or having a fully capable Python environment with lightning installed from the given branch?

[maliasadi]

Will this be strictly used for building wheels, building and running the test-suite, or having a fully capable Python environment with lightning installed from the given branch?

Hey @mlxd. Thanks for the review. The main benefit of having such Docker file is that it eases the process of building an image with Python and Lightning installed in it. This may help developers to create images of their work and test on different machines. However, as told you before, I'm not sure if it's worth to be added to the master branch; as its use case for me is on AWS only.

[mlxd]

Thanks @maliasadi . I think it will be useful for local testing and dev also.

I'd suggest one change, placing the Dockerfile into a directory called docker in the same structure as the PennyLane repo. Happy to approve it and let it be merged after this.