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 ./",
]