Add gradle caching function (#37)

gradle/Earthfile

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+VERSION --global-cache 0.7
+
+
+# GRADLE_GET_MOUNT_CACHE sets the following entries in the calling environment:
+# EARTHLY_GRADLE_USER_HOME_CACHE: Code of the mount cache for the gradle home.
+# EARTHLY_GRADLE_PROJECT_CACHE: Code of the mount cache for the .gradle folder.
+# Example:
+#   DO gradle+GRADLE_GET_MOUNT_CACHE
+#   RUN --mount=$EARTHLY_GRADLE_USER_HOME_CACHE --mount=$EARTHLY_GRADLE_PROJECT_CACHE gradle --no-daemon build
+# Arguments:
+#   - cache_prefix: To be used in both caches. By default: ${EARTHLY_TARGET_PROJECT_NO_TAG}#${OS_RELEASE}#earthly-gradle-cache
+GRADLE_GET_MOUNT_CACHE:
+    COMMAND
+    ARG EARTHLY_TARGET_PROJECT_NO_TAG
+    ARG cache_prefix = "${EARTHLY_TARGET_PROJECT_NO_TAG}#earthly-gradle-cache"
+    IF [ "$GRADLE_USER_HOME" = "" ]
+        ENV GRADLE_USER_HOME="$HOME/.gradle"
+    END
+    ENV EARTHLY_GRADLE_CACHE_PREFIX=$cache_prefix
+    ENV EARTHLY_GRADLE_USER_HOME_CACHE="type=cache,mode=0777,id=$cache_prefix,sharing=shared,target=$GRADLE_USER_HOME"
+    ENV EARTHLY_GRADLE_PROJECT_CACHE="type=cache,mode=0777,id=${cache_prefix}#project,sharing=shared,target=.gradle"

gradle/README.md

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+# lib/gradle
+
+Earthly's official collection of Gradle [functions](https://docs.earthly.dev/docs/guides/functions).
+
+First, import the library up in your Earthfile:
+```earthfile
+VERSION --global-cache 0.7
+IMPORT github.com/earthly/lib/gradle:<version/commit> AS gradle
+```
+> :warning: Due to [this issue](https://github.com/earthly/earthly/issues/3490), make sure to enable `--global-cache` in the calling Earthfile, as shown above.
+
+## +GRADLE_GET_MOUNT_CACHE
+
+This function sets the following entries in the calling environment, so they can be used later to parametrize two global mount caches in `RUN` commands:
+- `$EARTHLY_GRADLE_USER_HOME_CACHE`: Code of the mount cache for the [gradle user home](https://docs.gradle.org/current/userguide/directory_layout.html#dir:gradle_user_home) directory.
+- `$EARTHLY_GRADLE_PROJECT_CACHE`: Code of the mount cache for the [gradle project root](https://docs.gradle.org/current/userguide/directory_layout.html#dir:project_root) directory.
+
+### Arguments
+- `cache_prefix`: To be used in both caches. By default: `${EARTHLY_TARGET_PROJECT_NO_TAG}#${OS_RELEASE}#earthly-gradle-cache`
+
+### Example
+```earthly
+DO gradle+GRADLE_GET_MOUNT_CACHE
+RUN --mount=$EARTHLY_GRADLE_USER_HOME_CACHE --mount=$EARTHLY_GRADLE_PROJECT_CACHE gradle --no-daemon build
+```
+
+### Scoping
+
+If `cache_prefix` is not set, the two global caches have an Earthfile-scope, that is, they are not shared with other Earthfiles.
+Otherwise, all Earthfiles using the same `cache_prefix` will share the cache mounts.
+
+## Example
+See [earthly-intellij-plugin](https://github.com/earthly/earthly-intellij-plugin/blob/main/Earthfile) for a complete example.