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