How do I add a user when I’m using Alpine as a base image?

Alpine uses the command adduser and addgroup for creating users and groups (rather than useradd and usergroup).

FROM alpine:latest

# Create a group and user
RUN addgroup -S appgroup && adduser -S appuser -G appgroup

# Tell docker that all future commands should run as the appuser user
USER appuser

The flags for adduser are:

Usage: adduser [OPTIONS] USER [GROUP]

Create new user, or add USER to GROUP

        -h DIR          Home directory
        -g GECOS        GECOS field
        -s SHELL        Login shell
        -G GRP          Group
        -S              Create a system user
        -D              Don't assign a password
        -H              Don't create home directory
        -u UID          User id
        -k SKEL         Skeleton directory (/etc/skel)

Add new user official docs