Combining multiple fixes

[ianfixes]

• Supercedes preserve existing environment variable #4
• Supercedes Fix the silent timeout error when netcat is not installed #6
• Supercedes Add strict option #8
• Supercedes Namespacing local variables to prevent collision with environment. mrako/wait-for#5
• Supercedes add a backup for the environment variables named TIMEOUT, QUIT, `… mrako/wait-for#8
• Fixes Check if nc is available or give a helpful error mrako/wait-for#2
• Fixes The effect of the timeout option might surprise users mrako/wait-for#3 including special handling for OSX
• Fixes Using variable named PORT creates issues mrako/wait-for#4
• Fixes $cmdname not defined mrako/wait-for#7

Overall, 2 tests have been added and the CI now runs them both inside and outside the docker container.

[victordidenko]

Took me a while to find out, why my application uses mysql port to bind... And this one is still not merged 😢