Release arm64-compatible images

[tobybellwood]

This PR will release all uselagoon images in a multi-arch format, compatible with amd64 and arm64 architectures (M1 mac, Raspberry Pi 4 etc)

There are a few images that are currently non-functional outside of amd64:

• Elasticsearch 6 (an xpack plugin contains legacy x64 code)
• Varnish 6 (libvmod-dynamic doesn't cleanly compile)
• Mongo (built from alpine 3.8, non-arm64-compatible)

In addition, the following modifications have been made:

• the version of envplate used has been forked to github.com/amazeeio/envplate and turned into a multiarch docker image
• the version of go-crond has been updated to include the upstream changes, and consumed as a multi-arch Dockerfile
• the PHP monitoring and performance tools Xdebug, New Relic, and Blackfire have been updated to use the correct binary for the architecture being built
• the Elasticsearch 7 images have been upgraded to the earliest available multi-arch upstream
• the version of tini used in non-alpine builds has been made multi-arch compatible

closes #88 #320 #150