Remove building from running time

[krzema12]

Goals:

1. Save ~2 minutes of actions' running time (from user's perspective). Now the image is built every time the action is used:

   github-actions-typing/Dockerfile

   Lines 1 to 6 in fde861a

   	FROM gradle:8.4.0-jdk17 AS build
   	COPY --chown=gradle:gradle . /src
   	WORKDIR /src
   	RUN gradle build --no-daemon
   	RUN rm -rf dist
   	RUN unzip build/distributions/github-actions-typing.zip -d dist

2. Eliminate risk of failures during image building in actions' runtime, like:

  * What went wrong:
  Error resolving plugin [id: 'org.jetbrains.kotlin.jvm', version: '1.7.0']
  > Could not resolve all dependencies for configuration 'detachedConfiguration1'.
     > Could not determine artifacts for org.jetbrains.kotlin.jvm:org.jetbrains.kotlin.jvm.gradle.plugin:1.7.0
        > Could not get resource 'https://plugins.gradle.org/m2/org/jetbrains/kotlin/jvm/org.jetbrains.kotlin.jvm.gradle.plugin/1.7.0/org.jetbrains.kotlin.jvm.gradle.plugin-1.7.0.jar'.
           > Could not HEAD 'https://plugins.gradle.org/m2/org/jetbrains/kotlin/jvm/org.jetbrains.kotlin.jvm.gradle.plugin/1.7.0/org.jetbrains.kotlin.jvm.gradle.plugin-1.7.0.jar'.
              > Read timed out

The biggest challenge will be to ensure that if the user uses this action in a certain version/commit hash/branch, then the right Docker image is used.

[krzema12]

Idea: keep compiled JARs in the source code. Not perfect, but pragmatic. It's a frequent practice in TS-based actions. See the template: https://github.com/actions/typescript-action/tree/main/dist

In CI, validate that given source code produces given JARs. May be tedious when it comes to updating the deps, but let's see.

[krzema12]

How to check in CI that something changed or was added and is untracked by git:

git add .
git diff --cached --exit-code
echo $?