Consider switching to GitHub Actions for automated Docker Hub builds

[dennisameling]

As discussed in Slack (https://mautic.slack.com/archives/C0DMR1DRS/p1628762987006700), Docker has a paid Automated Builds feature. In the past something was set up by @luizeof but I'm not sure if that was leveraging this feature or something different:

As suggested by @RCheesley, it is possible to use GitHub Actions for pushing builds to Docker Hub (see this article). I'm happy to set that up, but first will need some info on the existing integration before I can move forward. Otherwise I might potentially break the existing integration or cause duplicate builds.

Required actions if we want to move to GH Actions:

• [ ] Get feedback from @luizeof on existing integration and how we can turn that off
• [x] Have @RCheesley be the account owner for Mautic Docker Hub and share credentials with me that GH Actions can use
• [ ] Me setting up the actual pipeline 😉

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[RCheesley]

I have access to DockerHub so I can sort out that side of it! Over to you @luizeof!

[mollux]

closing this issue as this was implemented in #238