Security vulnerability in current strapi/base

[iapain]

Latest strapi/base is based on older node-14 which is based on alpine-3.11.11 which contain a critical security vulnerability: CVE-2021-3711

Resolution: Update to node:14.7.6

[ChristianHeimke]

@iapain as I wrote here: #318 (comment) strapi docker images didn't support rebuilding. If you need to fix the issue you have to rebuild it on your own.
I made a PR #319 to use github actions - they build the images with the latest base images, maybe sometimes it will be used.