abstract out docker cmd, use sudo

[hongooi73]

This PR does a couple of things:

• Collects all the various system calls to run docker into one function docker
• docker then checks the OS type, and uses sudo to run docker if required.

Not (yet) implemented: using processx instead of system, sudo passwords

[hongooi73]

I also haven't touched the tests, they may need to be modified.

[codecov-io]

Codecov Report

Merging #7 into master will decrease coverage by 0.66%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master       #7      +/-   ##
==========================================
- Coverage   97.41%   96.75%   -0.67%     
==========================================
  Files           6        7       +1     
  Lines         271      277       +6     
==========================================
+ Hits          264      268       +4     
- Misses          7        9       +2

Impacted Files	Coverage Δ
R/docker_run.R	95.16% <100%> (ø)	⬆️
R/dockerfile_build.R	80.95% <100%> (ø)	⬆️
R/check_docker.R	100% <100%> (ø)	⬆️
R/docker_conn.R	100% <100%> (ø)	⬆️
R/docker.R	71.42% <71.42%> (ø)

[thebioengineer]

I like this solution and it removes the need to type "docker" at the beginning of each system call. Lets continue to think on how to resolve the sudo issue, look into processx, and adding tests.