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. The following are all valid: {@code user}, {@code uid},
{@code user:group}, {@code uid:gid}, {@code uid:group}, {@code user:gid}.

@param user the username/UID and optionally the groupname/GID
@return this
jib-core/src/main/java/com/google/cloud/tools/jib/configuration/ContainerConfiguration.java