Add a Dockerfile that includes all the tools needed to run tests

[nodivbyzero]

To run the go test command, this package requires the following tools to be installed:

• python
• twine
• cargo
• gpg
• pip
• wheel

This PR introduces a Dockerfile that includes all the necessary tools.

[jonyoder]

Can you update the README with steps to build and use the docker image? If it's complicated, it might be worth adding a Makefile, but I'm ok either way.

[jonyoder]

Thanks for adding the instructions. This looks good to me, but the tests are failing when I try to run them:

--- FAIL: TestParse (14.70s)
--- FAIL: TestParse/pytest_signed_tarball (0.51s)
parse_test.go:447:
Error Trace:    /python-distribution-parser/parse_test.go:447
Error:          Received unexpected error:
exit status 2
Test:           TestParse/pytest_signed_tarball
--- FAIL: TestParse/pytest_signed_wheel (0.51s)
parse_test.go:447:
Error Trace:    /python-distribution-parser/parse_test.go:447
Error:          Received unexpected error:
exit status 2
Test:           TestParse/pytest_signed_wheel
FAIL
FAIL    github.com/rstudio/python-distribution-parser   14.705s
FAIL

[nodivbyzero]

@jonyoder
As you can see, the CI passed, and I was able to run the tests successfully on my mac.
Could you try deleting the testdata folder and re-testing?

[jonyoder]

Feel free to merge it. No use holding it up since CI is passing.