Fix Travis CI issue with Docker server API version mismatch

Travis CI is using a version of Docker server that supports API version
1.38.  However, the Python PYPI package `docker` version 4.3 was
recently released which uses Docker server API version 1.39, and there
is apparently no way to tell molecule to configure the `docker` Python
API to use a different API.  The solution is to install the latest 4.2.x
version of the `docker` Python PYPI package.

.travis/runtox

@@ -16,6 +16,24 @@ ME=$(basename $0)
 SCRIPTDIR=$(readlink -f $(dirname $0))
 BANNERSIZE=90
 
+# LSR_MOLECULE_DOCKER_VERSION is the version string to pass
+# to pip install - this is usually a comparison operator
+# followed by a version number e.g. '<4.3' but in reality
+# may be anything that pip allows to follow a package name
+# in an install specification
+if [ -z "${LSR_MOLECULE_DOCKER_VERSION:-}" ]; then
+  if type docker > /dev/null 2>&1; then
+    DOCKER_SERVER_API_VERSION=$(docker version --format '{{.Server.APIVersion}}')
+    case "$DOCKER_SERVER_API_VERSION" in
+    1.3[0-8]|1.[0-2]*) LSR_MOLECULE_DOCKER_VERSION='<4.3';;
+    *) LSR_MOLECULE_DOCKER_VERSION="";;
+    esac
+  else
+    LSR_MOLECULE_DOCKER_VERSION=""
+  fi
+fi
+export LSR_MOLECULE_DOCKER_VERSION
+
 . ${SCRIPTDIR}/utils.sh
 . ${SCRIPTDIR}/config.sh
 

tox.ini

@@ -109,7 +109,7 @@ commands =
 changedir = {toxinidir}
 deps =
     {env:LSR_ANSIBLE_DEP:ansible}
-    docker
+    docker{env:LSR_MOLECULE_DOCKER_VERSION:}
     molecule<3
     selinux
     -rmolecule_extra_requirements.txt