Include dashbootstrap in R docker image

[joelostblom]

There is a version of dashbootstrap for R and it would be convenient if this was included in the dash docker image for deployment on heroku.

Including here would also be great as it is often used for file path consistency.

[rpkyle]

Thanks for the suggestion @joelostblom! I don't have a problem with this myself, especially given the utility of the package, though dashBootstrapComponents doesn't take long to install (unlike some of the other dependencies we roll in prior to deployment).

We'll keep it in mind when we next update the buildpack.