Docker build update

[dopamane]

Fixes manifest unknown error when running ./docker/build.sh:

Sending build context to Docker daemon    752MB
Step 1/21 : ARG K_COMMIT
Step 2/21 : FROM runtimeverificationinc/kframework-k:ubuntu-bionic-${K_COMMIT}
manifest for runtimeverificationinc/kframework-k:ubuntu-bionic-v5.1.3 not found: manifest unknown: manifest unknown

Use the newer docker hub tag format by removing prefix "v" from k release version with cut -c2- in ./docker/build.sh. I also removed the old tag formatter cut --delimiter="-" --field="2".

Additionally, in the readme, specify kore as a make target in order to build kore-exec, kore-repl, and kore-parser which are all needed when running ./docker/run.sh make test. Previously, ./docker/run.sh make only built kore-exec.

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Review checklist

The author performs the actions on the checklist. The reviewer evaluates the work and checks the boxes as they are completed.

• Summary. Write a summary of the changes. Explain what you did to fix the issue, and why you did it. Present the changes in a logical order. Instead of writing a summary in the pull request, you may push a clean Git history.
• Documentation. Write documentation for new functions. Update documentation for functions that changed, or complete documentation where it is missing.
• Tests. Write unit tests for every change. Write the unit tests that were missing before the changes. Include any examples from the reported issue as integration tests.
• Clean up. The changes are already clean. Clean up anything near the changes that you noticed while working. This does not mean only spatially near the changes, but logically near: any code that interacts with the changes!