chore(build): upgrade builder image to gradle:7.6.1-jdk17

[difranca]

Kayenta build using Dockerfile is currently broken:

58.02 FAILURE: Build failed with an exception.
58.02 
58.02 * Where:
58.02 Build file '/tmp/workdir/build.gradle' line: 63
58.02 
58.02 * What went wrong:
58.02 A problem occurred evaluating root project 'kayenta'.
58.02 > Could not get unknown property 'javaToolchains' for task ':kayenta-atlas:test' of type org.gradle.api.tasks.testing.Test.

Gradle version was bumped to 7.6.1 in #967, but the base image for the build stage was not updated.
Changing it to gradle:7.6.1-jdk17 fixes the build.