docker-compose.yml: rework tox

Re-work the tox deployment so that we mount the application directory as
a read-only volume rather than copying the file in when building the
image. This has the following advantages:

1. We do not build a new image each time we run tests. (This is nicer to
   those of us with finite disk space.)
2. Running tests is slightly faster since a new image is not being
   built.
3. The application directory is mounted read-only so we catch code which
   accidentally writes to the current directory.
4. We can use the upstream tox image rather than rolling our own derived
   image.

In order to make to happy to run in a read-only environment, we specify
that a) it does not run sdist to write an .egg-info directory and b)
that coverage data is stored in /tmp rather than the application
directory.

Dockerfile

-FROM themattrix/tox-base
-ADD . .
-RUN tox

README.md

@@ -159,5 +159,5 @@ Tox is configured to run on a container with a matrix execution of different ver
 It will also show the coverage and any possible PEP8 violations.
 
 ```shell
-$ docker-compose up --build tox
+$ docker-compose up
 ```

README.rst

@@ -199,4 +199,4 @@ It will also show the coverage and any possible PEP8 violations.
 
 .. code:: shell
 
-        $ docker-compose up --build tox
+        $ docker-compose up

docker-compose.yml

@@ -6,14 +6,15 @@
 version: '3.2'
 services:
   tox:
-    build:
-      context: .
-      dockerfile: Dockerfile
+    image: themattrix/tox-base
+    entrypoint: tox
     environment:
       - TOXINI_WORK_DIR=/tmp/tox-data/work
       - TOXINI_ARTEFACT_DIR=/tmp/tox-data/artefacts
+      - TOXINI_COVERAGE_FILE=/tmp/tox-coverage
     volumes:
       - tox-data:/tmp/tox-data
+      - ./:/app:ro
 
 volumes:
   # A persistent volume for tox to store its stuff. This allows caching of

tox.ini

@@ -17,6 +17,9 @@ envlist =
     flake8
 # Allow overriding toxworkdir via environment variable
 toxworkdir={env:TOXINI_WORK_DIR:{toxinidir}/.tox}
+# Do not attempt to create .egg-info directories in the application root as it
+# is mounted as a read-only volume.
+skipsdist=true
 
 # The "_vars" section is ignored by tox but we place some useful shared
 # variables in it to avoid needless repetition.
@@ -27,6 +30,10 @@ toxworkdir={env:TOXINI_WORK_DIR:{toxinidir}/.tox}
 build_root={env:TOXINI_ARTEFACT_DIR:{toxinidir}/build}
 
 [testenv]
+setenv=
+#   Override the coverage dtaa file location since the application root is
+#   mounted read-only.
+    COVERAGE_FILE={env:TOXINI_COVERAGE_FILE:{toxinidir}/.coverage}
 # Additional dependencies
 deps=
     coverage