Don't remove TABs that are inside the command

[SvenDowideit]

only remove those that are at the beginning, end and separator

(Closes #2315) due to #522 (i think)

[SvenDowideit]

ok, so when I forced everything to run with my newly build docker bundle, this breaks. so going back to learning :)

[SvenDowideit]

and the above commit then breaks further code that assumes tabs have been replaced using spaces, notably

Step 11 : RUN cd /goroot/src && ./make.bash
 ---> Using cache
 ---> 635840d198e5
Step 12 : ENV GOROOT    /goroot
Error build: Invalid ENV format
Invalid ENV format

which again, should not be done to all tabs :/

clearly, all this gets better once we have a real parser for Dockerfiles #2266, #2225 , #2105

[SvenDowideit]

#2266 should resolve these issues

[shykes]

@SvenDowideit do you still want us to review this?

[SvenDowideit]

@shykes I guess not, I assume that #2225 will get merged eventually