There is a Copy() function, but it is not exported: https://github.com/o2r-project/containerit/blob/master/R/Class-Copy.R

After exporting that, I'm happy to add a paramter to main method: dockerfile(.., copy = list(Copy("from", "to"), Copy("from2", "to2")).
The tricky thing is where to put the custom COPY statements. The later they come in the file, the less impact they have on build caching, so I'd say put them right before the ENTRYPOINT.
Do you agree?

I'll take a stab at this, possibly next week and also fix #172 (Don't you run into that error? That is why my example below does not filter base image packages..).
Until then, you can append further instructions with addInstruction(..):

> docker <- suppressWarnings(dockerfile(expression("library(plumber)"),
     image = "trestletech/plumber",
     offline = FALSE,
     cmd = Cmd("api.R"),
     maintainer = NULL,
     container_workdir = NULL,
     entrypoint = Entrypoint("R",
                   params = list("-e",
                                 "pr <- plumber::plumb()")),
     filter_baseimage_pkgs = FALSE))

> addInstruction(docker) <- containerit:::Copy("host", "container")
> print(docker)
FROM trestletech/plumber
RUN export DEBIAN_FRONTEND=noninteractive; apt-get -y update \
  && apt-get install -y git-core \
	libcurl4-openssl-dev \
	libssl-dev \
	make
RUN ["install2.r", "assertthat", "backports", "crayon", "curl", "desc", "digest", "fastmap", "formatR", "fs", "futile.logger", "futile.options", "htmltools", "httpuv", "jsonlite", "lambda.r", "later", "magrittr", "mime", "miniUI", "pillar", "pkgconfig", "promises", "R6", "Rcpp", "remotes", "rlang", "rprojroot", "semver", "shiny", "shinyFiles", "stevedore", "stringi", "stringr", "tibble", "xtable"]
RUN ["installGithub.r", "goldingn/versions@de231cb523aa9134a1bd4e947c8204a7a08f3daa"]
COPY ["host", "container"]
ENTRYPOINT ["R", "-e", "pr <- plumber::plumb()"]
CMD ["api.R"]

The tricky thing is where to put the custom COPY statements. The later they come in the file, the less impact they have on build caching, so I'd say put them right before the ENTRYPOINT.

Yes that is where I tend to put them.

I'll take a stab at this, possibly next week and also fix #172 (Don't you run into that error? That is why my example below does not filter base image packages..).

Ah work in progress so not come across it yet, not tried it beyond test scripts without heavy library dependencies yet.

Its for a Cloud Build package which incidentally is my preferred alternative to docker_build now - https://github.com/MarkEdmondson1234/cloudRunner