Improve and fix tests (#36)

* Parameterize test images.

* Use different images, similar to ansible/ansible#72698.

tests/integration/targets/docker_image/templates/Dockerfile

@@ -0,0 +1,3 @@
+FROM {{ docker_test_image_busybox }}
+ENV foo /bar
+WORKDIR ${foo}

tests/integration/targets/docker_image/templates/EtcHostsDockerfile

@@ -0,0 +1,3 @@
+FROM {{ docker_test_image_busybox }}
+# This should fail building if docker cannot resolve some-custom-host
+RUN ping -c1 some-custom-host

tests/integration/targets/docker_image/templates/MyDockerfile

@@ -0,0 +1,5 @@
+FROM {{ docker_test_image_alpine }}
+ENV INSTALL_PATH /newdata
+RUN mkdir -p $INSTALL_PATH
+
+WORKDIR $INSTALL_PATH

tests/integration/targets/docker_image/templates/StagedDockerfile

@@ -0,0 +1,7 @@
+FROM {{ docker_test_image_busybox }} AS first
+ENV dir /first
+WORKDIR ${dir}
+
+FROM {{ docker_test_image_busybox }} AS second
+ENV dir /second
+WORKDIR ${dir}