[docker] How do I add a user when I'm using Alpine as a base image?

The commands are adduser and addgroup.

Here's a template for Docker you can use in busybox environments (alpine) as well as Debian-based environments (Ubuntu, etc.):

ENV USER=docker
ENV UID=12345
ENV GID=23456

RUN adduser \
    --disabled-password \
    --gecos "" \
    --home "$(pwd)" \
    --ingroup "$USER" \
    --no-create-home \
    --uid "$UID" \
    "$USER"

Note the following:

• --disabled-password prevents prompt for a password
• --gecos "" circumvents the prompt for "Full Name" etc. on Debian-based systems
• --home "$(pwd)" sets the user's home to the WORKDIR. You may not want this.
• --no-create-home prevents cruft getting copied into the directory from /etc/skel

The usage description for these applications is missing the long flags present in the code for adduser and addgroup.

The following long-form flags should work both in alpine as well as debian-derivatives:

adduser

BusyBox v1.28.4 (2018-05-30 10:45:57 UTC) multi-call binary.

Usage: adduser [OPTIONS] USER [GROUP]

Create new user, or add USER to GROUP

        --home DIR           Home directory
        --gecos GECOS        GECOS field
        --shell SHELL        Login shell
        --ingroup GRP        Group (by name)
        --system             Create a system user
        --disabled-password  Don't assign a password
        --no-create-home     Don't create home directory
        --uid UID            User id

One thing to note is that if --ingroup isn't set then the GID is assigned to match the UID. If the GID corresponding to the provided UID already exists adduser will fail.

addgroup

BusyBox v1.28.4 (2018-05-30 10:45:57 UTC) multi-call binary.

Usage: addgroup [-g GID] [-S] [USER] GROUP

Add a group or add a user to a group

        --gid GID  Group id
        --system   Create a system group

I discovered all of this while trying to write my own alternative to the fixuid project for running containers as the hosts UID/GID.

My entrypoint helper script can be found on GitHub.

The intent is to prepend that script as the first argument to ENTRYPOINT which should cause Docker to infer UID and GID from a relevant bind mount.

An environment variable "TEMPLATE" may be required to determine where the permissions should be inferred from.

(At the time of writing I don't have documentation for my script. It's still on the todo list!!)