Using exec_filter within Docker container

I am moving my fluentd instance into a container and running into issues with the exec_filter. My fluent.conf file uses the exec_filter to execute a python script. When I deploy my container I get the following error:

[error]: #0 unexpected error error_class=Errno::ENOENT error="No such file or directory - python3"

My match block with the exec_filter is below.

<match logs>
  @type exec_filter
  tag new_logs
  command python3 /fluentd/etc/python_processing/fluent.py

  <format>
    @type json
  </format>

  <parse>
    @type json
  </parse>

  <buffer>
    @type file
    path /var/log/fluent/execbuffer
    flush_interval 1m
  </buffer>

  @log_level debug
</match>

And my Dockerfile.

FROM python:3
WORKDIR /app
COPY requirements.txt /app
RUN pip install -r /app/requirements.txt
ENTRYPOINT [ "python3" ]

FROM alpine:3.9
LABEL maintainer "Fluentd developers <fluentd@googlegroups.com>"
LABEL Description="Fluentd docker image" Vendor="Fluent Organization" Version="1.9.1"

RUN apk update \
 && apk add --no-cache \
        ca-certificates \
        ruby ruby-irb ruby-etc ruby-webrick \
        tini \
 && apk add --no-cache --virtual .build-deps \
        build-base linux-headers \
        ruby-dev gnupg \
 && echo 'gem: --no-document' >> /etc/gemrc \
 && gem install oj -v 3.8.1 \
 && gem install json -v 2.3.0 \
 && gem install async-http -v 0.50.0 \
 && gem install ext_monitor -v 0.1.2 \
 && gem install fluentd -v 1.9.1 \
 && gem install bigdecimal -v 1.4.4 \
 && apk del .build-deps \
 && rm -rf /tmp/* /var/tmp/* /usr/lib/ruby/gems/*/cache/*.gem

RUN addgroup -S fluent && adduser -S -g fluent fluent \
    # for log storage (maybe shared with host)
    && mkdir -p /fluentd/log \
    # configuration/plugins path (default: copied from .)
    && mkdir -p /fluentd/etc /fluentd/plugins \
    && chown -R fluent /fluentd && chgrp -R fluent /fluentd \
    && mkdir -p /var/log/fluent/execbuffer \
    && mkdir -p /fluentd/etc/python_processing

COPY fluent.conf /fluentd/etc/
COPY entrypoint.sh /bin/
COPY parser_multi_format.rb /fluentd/plugins/

COPY fluent.py /fluentd/etc/python_processing/


ENV FLUENTD_CONF="fluent.conf"

ENV LD_PRELOAD=""
EXPOSE 24224 5140

USER fluent
ENTRYPOINT ["tini",  "--", "/bin/entrypoint.sh"]
CMD ["fluentd"]

Any ideas?

I have resolved this issue on my own and it can be closed now.

Solution: Python needs to be installed into the Alpine environment using the apk commands.