/bin/sh: 1: apk: not found while creating docker image

I have the below Dockerfile for zookeeper and I am trying to create an image for it, but it is giving me an error. I have recently started working with Docker, and started playing with a Zookeeper setup, so I am not able to understand. What does this error mean?

FROM ubuntu:trusty
MAINTAINER David

RUN apt-get update && apt-get install -y openjdk-7-jre-headless wget
ENV JAVA_HOME /usr/lib/jvm/java-7-openjdk-amd64

# Install required packages
RUN apk add --no-cache \
	bash \
	su-exec

ENV ZOO_USER=zookeeper \
	ZOO_CONF_DIR=/conf \
	ZOO_DATA_DIR=/data \
	ZOO_DATA_LOG_DIR=/datalog \
	ZOO_PORT=2181 \
	ZOO_TICK_TIME=2000 \
	ZOO_INIT_LIMIT=5 \
	ZOO_SYNC_LIMIT=2

# Add a user and make dirs
RUN set -x \
	&& adduser -D "$ZOO_USER" \
	&& mkdir -p "$ZOO_DATA_LOG_DIR" "$ZOO_DATA_DIR" "$ZOO_CONF_DIR" \
	&& chown "$ZOO_USER:$ZOO_USER" "$ZOO_DATA_LOG_DIR" "$ZOO_DATA_DIR" "$ZOO_CONF_DIR"

ARG GPG_KEY=C823E3E5B12AF29C67F81976F5CECB3CB5E9BD2D
ARG DISTRO_NAME=zookeeper-3.4.9

# Download Apache Zookeeper, verify its PGP signature, untar and clean up
RUN set -x \
	&& apk add --no-cache --virtual .build-deps \
		gnupg \
	&& wget -q "http://www.apache.org/dist/zookeeper/$DISTRO_NAME/$DISTRO_NAME.tar.gz" \
	&& wget -q "http://www.apache.org/dist/zookeeper/$DISTRO_NAME/$DISTRO_NAME.tar.gz.asc" \
	&& export GNUPGHOME="$(mktemp -d)" \
	&& gpg --keyserver ha.pool.sks-keyservers.net --recv-key "$GPG_KEY" \
	&& gpg --batch --verify "$DISTRO_NAME.tar.gz.asc" "$DISTRO_NAME.tar.gz" \
	&& tar -xzf "$DISTRO_NAME.tar.gz" \
	&& mv "$DISTRO_NAME/conf/"* "$ZOO_CONF_DIR" \
	&& rm -r "$GNUPGHOME" "$DISTRO_NAME.tar.gz" "$DISTRO_NAME.tar.gz.asc" \
	&& apk del .build-deps

WORKDIR $DISTRO_NAME
VOLUME ["$ZOO_DATA_DIR", "$ZOO_DATA_LOG_DIR"]

EXPOSE $ZOO_PORT 2888 3888

ENV PATH=$PATH:/$DISTRO_NAME/bin \
	ZOOCFGDIR=$ZOO_CONF_DIR

COPY docker-entrypoint.sh /
ENTRYPOINT ["/docker-entrypoint.sh"]
CMD ["zkServer.sh", "start-foreground"]

Below is the error I got:

Step 4 : ENV JAVA_HOME /usr/lib/jvm/java-7-openjdk-amd64
 ---> Running in a49507cb9725
 ---> 77b37883caf4
Removing intermediate container a49507cb9725
Step 5 : RUN apk add --no-cache     bash     su-exec
 ---> Running in a4fd76a644cf
/bin/sh: 1: apk: not found
The command '/bin/sh -c apk add --no-cache     bash     su-exec' returned a non-zero code: 127

Am I doing anything wrong here? Why is apk not found?

As larsks mentions, apk is for Alpine distributions and you selected FROM ubuntu:trusty which is Debian based with the apt-get command. Change your FROM line to FROM alpine:3.4 to switch to the Alpine based image with apk support.

Quite late to the party. I'll put down what worked for me.

As john rightly pointed out that apk is package manager for alpine distributions, for ubuntu image, we need to use apt-get:

FROM ubuntu:trusty
RUN apt-get update && apt-get install -y tini

Otherwise Alpine base image can be used to run apk commands:

FROM python:3.7-alpine3.12
RUN apk add --no-cache tini