add .run_line_wrap helper function to wrap long CMD lines in dockerfiles

[e-kotov]

Currently the dockerize() function produces rather unfriendly long lines of RUN commands. For example:

RUN apt-get update -qq && apt-get install -y libpcre3-dev zlib1g-dev pkg-config libcurl4-openssl-dev && apt-get install -y libcurl4-openssl-dev libicu-dev libssl-dev make zlib1g-dev

With the addition of the new internal (non-exported) helper function .run_line_wrap() this becomes a more conventional and human readable:

RUN apt-get update -qq \
	&& apt-get install -y libpcre3-dev zlib1g-dev pkg-config libcurl4-openssl-dev \
	&& apt-get install -y libcurl4-openssl-dev libicu-dev libssl-dev make zlib1g-dev

[codecov-commenter]

Codecov Report

Merging #128 (45fe555) into v0.3 (0866aad) will increase coverage by 0.00%.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             v0.3     #128   +/-   ##
=======================================
  Coverage   97.26%   97.27%           
=======================================
  Files          10       10           
  Lines         989      991    +2     
=======================================
+ Hits          962      964    +2     
  Misses         27       27           

Impacted Files	Coverage Δ
R/dockerfile.R	100.00% <100.00%> (ø)
R/installation.R	97.43% <100.00%> (+0.02%)	⬆️

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[chainsawriot]

@e-kotov Thanks a lot for the PR! I have provide the CR. Please let me know if you need help with the requested changes.

[e-kotov]

@chainsawriot I incorporated your review suggestions and written a test. I could not come up with the one that would use writeLines(). To me, it makes sense to check if all lines with && start with \t && and that there are no lines where there are two or more occurrences of &&.

[chainsawriot]

@e-kotov Thanks a lot and I will add you as a CTB.