Use docker contexts for fetching sources (where possible)

[nightkr]

Docs: https://docs.docker.com/build/bake/contexts/

This should let us get rid of a bunch of redundant downloading code, but more importantly make it easier to patch out components ("use this local jmx-exporter sources instead of downloading the pinned version from git").

[NickLarsenNZ]

This is already happening to a degree. Or at least when I look at the generated bake file, I see contexts.

Eg:

FROM stackable/image/statsd_exporter AS statsd_exporter-builder

  "target": {
    "airflow-2_10_2": {
      "context": ".",
      "contexts": {
        "stackable/image/statsd_exporter": "target:statsd_exporter-0_28_0",
        "stackable/image/vector": "target:vector-0_43_1"
      },