i get the same problem. Install by this way resolve the problem:

RUN curl -LkSso /usr/bin/mhsendmail 'https://github.com/mailhog/mhsendmail/releases/download/v0.2.0/mhsendmail_linux_amd64'&&
chmod 0755 /usr/bin/mhsendmail &&
echo 'sendmail_path = "/usr/bin/mhsendmail --smtp-addr=mailhog:1025"' > /usr/local/etc/php/php.ini;