How can I add a user with Dockerfile - the following does not work.

```
USER vault
WORKDIR /usr/local/bin/vault

```

My full Dockerfile:

```
FROM alpine:3.4
RUN apk update && apk add curl unzip
RUN useradd -ms /bin/bash vault

USER vault
WORKDIR /usr/local/bin/vault
ADD /vault.hcl /etc/vault/vault.hcl

RUN curl -SL https://releases.hashicorp.com/vault/0.5.0/vault_0.5.0_linux_amd64.zip > vault.zip
RUN unzip vault.zip -d /usr/local/bin && rm vault.zip

```