public Builder setUser(@Nullable String user) {
      this.user = user;
      return this;
    }
Sets the user and group to run the container as. {@code user} can be a username or UID along
with an optional groupname or GID. {@code null} signals to use the base image value.

<p>The following are valid formats for {@code user}

<ul>
  <li>{@code user}
  <li>{@code uid}
  <li>{@code :group}
  <li>{@code :gid}
  <li>{@code user:group}
  <li>{@code uid:gid}
  <li>{@code uid:group}
  <li>{@code user:gid}
</ul>

@param user the user to run the container as
@return this
jib-build-plan/src/main/java/com/google/cloud/tools/jib/api/buildplan/ContainerBuildPlan.java