upgrade to Alpine Linux 3.15 [BAC-2929]

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BAC-2929 Update Alpine Linux

Previously, we were using node:12.18.3-alpine, which included Node 12.18.3 and Alpine Linux 3.11. Alpine Linux 3.11 has a number of critical vulnerabilities and exposures and security support ended in November 2021.

This PR upgrades us to use node:12.22.12-alpine3.15, which includes Node 12.22.12 and Alpine Linux 3.15. Node 12.22 is the earliest minor version of Node 12 that is available with Alpine Linux 3.15, and Alpine Linux 3.15 has no known vulnerabilities as of July 2022. Security support will continue for Alpine Linux 3.15 until November 2023.

The major changes between Node 12.18.3 and Node 12.22.12 are available in the CHANGELOG:

• Fixed security vulnerabilities
• Upgraded npm to 6.14.11
• Deprecated process.umask(), which we don't use
• Deprecated the legacy HTTP parser, which we don't use