How to add user with dockerfile?

Use useradd instead of its interactive adduser to add user.

RUN useradd -ms /bin/bash  vault

Below command will not create user .

USER vault
WORKDIR /usr/local/bin/vault

it will use vault user

please Refer Dockerfile User Documentation

The USER instruction sets the user name or UID to use when running the image and for any RUN, CMD and ENTRYPOINT instructions that follow it in the Dockerfile.

NOTE : Ensures that bash is the default shell.

If default shell is /bin/sh you can do like:

RUN ln -sf /bin/bash /bin/sh
RUN useradd -ms /bin/bash  vault

To add group and to associate a new user, use code below.

FROM <base image>
RUN groupadd -g 2000 go \
&& useradd -m -u 2001 -g go go
USER go

OR

RUN addgroup -g 1001 -S appuser && adduser -u 1001 -S appuser  -G appuser