Fix bug in publishing :latest tag to Docker Hub (#45)

There were two issues tied together here:
- The regex was failing because the version of `grep` on Circle needs the `-P` option for our expression.
- The regex fails (intentionally) for pre-release tags, but that was causing the step to fail, rather than switch branches and log that there was nothing to publish.

Author: Mr0grog

.circleci/config.yml

@@ -100,7 +100,13 @@ jobs:
           name: Publish :latest Tag If Final Release
           command: |
             # Skip tags with additional info after "v0.0.0", e.g. "v1.0.0a1"
-            FINAL_VERSION="$(echo "${CIRCLE_TAG}" | grep -E '^v\d+(\.\d+)+$')"
+            # NOTE: the version of grep on Circle needs -P (perl-style regex)
+            # instead of -E (extended regex) to support `\d`.
+            FINAL_VERSION="$(
+              echo "${CIRCLE_TAG}" |
+              grep -P '^v\d+(\.\d+)+$' ||
+              true
+            )"
             if [ -z "${FINAL_VERSION}" ]; then
               echo 'The current tag does not represent a final release!'
               echo 'Not publishing a ":latest" tag.'