RFR: 8282944: GHA: Add Alpine Linux x86_64 pre-integration check

[George Adams]

Adds Alpine build CI job to the GitHub actions submit.yml. I can add tests too if people think that would be useful but for now I've left it as just build.

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Commit messages:
 - fix sha256 command
 - add alpine github action

Changes: https://git.openjdk.java.net/jdk/pull/7771/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7771&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282944
  Stats: 107 lines in 2 files changed: 106 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7771.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7771/head:pull/7771

PR: https://git.openjdk.java.net/jdk/pull/7771