(dockerfile): initial dockerfile

Dockerfile

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+#
+# simple postfix smarthost smtp relay
+#
+
+FROM alpine:3.13
+
+# standardized labels
+LABEL author="Asif Bacchus <asif@bacchus.cloud>"
+LABEL maintainer="Asif Bacchus <asif@bacchus.cloud>"
+LABEL org.label-schema.schema-version="1.0"
+LABEL org.label-schema.docker.cmd=""
+LABEL org.label-schema.description="Simple postfix smarthost smtp mail relay on Alpine Linux."
+LABEL org.label-schema.url=""
+LABEL org.label-schema.usage=""
+LABEL org.label-schema.vcs-url=""
+
+# install mSMTP
+RUN apk --no-cache \
+    ca-certificates \
+    postfix \
+    bind-tools \
+    && rm -f /var/cache/apk/*
+
+# set environment variables
+ENV TZ=Etc/UTC
+ENV HOSTNAME=${HOSTNAME}
+ENV DOMAINNAME=${HOSTNAME#*.}
+ENV LOCAL_ENCRYPTION=false
+ENV SMARTHOST=""
+ENV SMARTHOST_PORT=587
+ENV SMARTHOST_USERNAME=""
+ENV SMARTHOST_PASSWORD=""
+ENV SMARTHOST_ENCRYPTION="OPTIONAL"
+
+# copy configuration snippets
+COPY main.cf.insert /tmp/main.cf.insert
+COPY entrypoint.sh /usr/local/bin/entrypoint.sh
+
+# set entrypoint and default command
+ENTRYPOINT [ "/usr/local/bin/entrypoint.sh" ]
+CMD [ "/usr/local/sbin/postfix", "start-fg" ]
+
+# set parameters, vendor, version and build-date labels
+LABEL org.label-schema.docker.params="TZ=Etc/UTC, HOSTNAME=HOSTNAME, DOMAINNAME=(derived from hostname), LOCAL_ENCRYPTION=false, SMARTHOST, SMARTHOST_PORT=587, SMARTHOST_USERNAME, SMARTHOST_PASSWORD, SMARTHOST_ENCRYPTION='OPTIONAL'"
+LABEL org.label-schema.vendor="Alpine 3.13, Postfix 3.5.10-r0"
+LABEL org.label-schema.version="0.1"
+ARG BUILD_DATE
+LABEL org.label-schema.build-date=${BUILD_DATE}