Improve efficiency of Dockerfile by taking better advantage of layering

Author: amotl

CHANGES.rst

@@ -6,14 +6,14 @@ mqttwarn changelog
 in progress
 ===========
 
-- [build] Fix unwanted cache hits when automatically building Docker images
+- [build] Fix unwanted cache hits when automatically building Docker images. Thanks, Gergő!
 
 
 2021-06-03 0.21.0
 =================
 - [xmpp] Add slixmpp plugin and documentation. Thanks, Remi!
 - [mysql] Fix unicode vs byte issue for Python 3. Thanks, Filip!
-- [http] Fix to send whole message. Thanks, Gergo!
+- [http] Fix to send whole message. Thanks, Gergő!
 - [slack] Replace ``slacker`` with ``slack-sdk``. Thanks, mcdamo!
 - [mysql] Fix specified port bug. Thanks, Hyman!
 - [launch] Add new service ``launch``. Flexible arguments for command,

Dockerfile

@@ -7,10 +7,6 @@
 #
 FROM python:3.9-slim-buster
 
-# Install mqttwarn
-COPY . /src
-RUN pip install /src
-
 # Create /etc/mqttwarn
 RUN mkdir -p /etc/mqttwarn
 WORKDIR /etc/mqttwarn
@@ -28,5 +24,9 @@ VOLUME ["/etc/mqttwarn"]
 # Set default configuration path
 ENV MQTTWARNINI="/etc/mqttwarn/mqttwarn.ini"
 
+# Install mqttwarn
+COPY . /src
+RUN pip install /src
+
 # Invoke program
 CMD mqttwarn