sendmail - not found in apk repositories

Hi
We are using Docker and want to install sendmail

Unfortunately it is not found in any repository I'm aware of

details:
This is our dockerfile:
================================================
FROM openjdk:8-jre-alpine

ENV SPRING_OUTPUT_ANSI_ENABLED=ALWAYS \
JHIPSTER_SLEEP=0 \
JAVA_OPTS=""

# add directly the war
ADD *.war /app.war
RUN apk add sendmail

EXPOSE 8080
CMD echo "The application will start in ${JHIPSTER_SLEEP}s..." && \
sleep ${JHIPSTER_SLEEP} && \
java ${JAVA_OPTS} -Djava.security.egd=file:/dev/./urandom -jar /app.war
================================================

the "RUN apk add sendmail" line gives the following error:

[4:22 PM] Eyal Kotler: ERROR: unsatisfiable constraints:
  sendmail (missing):
    required by: world[sendmail]
[4:23 PM] Eyal Kotler: cat /etc/*-release
NAME="Alpine Linux"
ID=alpine
VERSION_ID=3.7.0
PRETTY_NAME="Alpine Linux v3.7"
HOME_URL="http://alpinelinux.org"
BUG_REPORT_URL="http://bugs.alpinelinux.org"
cat: can't open '/etc/system-release': No such file or directory

any help is appreciated.