How to copy multiple files in one layer using a Dockerfile?

The following Dockerfile contains four COPY layers:

COPY README.md ./
COPY package.json ./
COPY gulpfile.js ./
COPY __BUILD_NUMBER ./

How to copy these files using one layer instead? The following was tried:

COPY [
    "__BUILD_NUMBER ./",
    "README.md ./",
    "gulpfile ./",
    "another_file ./",
]

COPY README.md package.json gulpfile.js __BUILD_NUMBER ./

or

COPY ["__BUILD_NUMBER", "README.md", "gulpfile", "another_file", "./"]

You can also use wildcard characters in the sourcefile specification. See the docs for a little more detail.

Directories are special! If you write

COPY dir1 dir2 ./

that actually works like

COPY dir1/* dir2/* ./

If you want to copy multiple directories (not their contents) under a destination directory in a single command, you'll need to set up the build context so that your source directories are under a common parent and then COPY that parent.

COPY <all> <the> <things> <last-arg-is-destination>

But here is an important excerpt from the docs:

If you have multiple Dockerfile steps that use different files from your context, COPY them individually, rather than all at once. This ensures that each step’s build cache is only invalidated (forcing the step to be re-run) if the specifically required files change.

https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#add-or-copy

It might be worth mentioning that you can also create a .dockerignore file, to exclude the files and directories that you don't want to copy:

https://docs.docker.com/engine/reference/builder/#dockerignore-file

Before the docker CLI sends the context to the docker daemon, it looks for a file named .dockerignore in the root directory of the context. If this file exists, the CLI modifies the context to exclude files and directories that match patterns in it. This helps to avoid unnecessarily sending large or sensitive files and directories to the daemon and potentially adding them to images using ADD or COPY.