Test buildx addition to multistage docker build action

[bquan0]

Following this conversation, I tried out a version of the Firehed multistage docker build action that uses buildx. I set up the Dockerfile to have stages 1, 2a, 2b, 3a, and 3b, where 2a and 3a depend on 1, and 2b and 3b depend on 2a and 3a respectively. The action seems to have the key buildx functionalities such as:

• The action now skips over stages that it doesn't need to build. In my tests, I made 3b the final stage, and from this run (which was the first run, so the one that actually built everything instead of pulling from the cache), we can see that it only built stages 1, 3a, and 3b.
• It's now not necessary to "import" args into every stage where they are used. Instead, you only need to "import" them in one stage, and subsequent stages built off of that stage will have access to the arg. See this workflow run where I "imported" the build arg in stage1 and it's still available in stage3b.

Overall, I think the changes to the multistage action to make it use buildx are working properly.

[coveralls]

Pull Request Test Coverage Report for Build 4452483034

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 82.222%

---

Totals
Change from base Build 3932885115:	0.0%
Covered Lines:	74
Relevant Lines:	90

---

💛 - Coveralls

[gonuke]

Thanks @bquan0 - great to see this working.