Clarify ordering in Dockerfile for Fly deployments

[APB9785]

If the step

RUN mix esbuild.install --if-missing

comes before

RUN mix assets.deploy

then the app can fail to boot with an error like:

2025-02-14T18:37:34Z app[17815ee1c522d8] lhr [info]** (Beacon.LoaderError) esbuild binary not found or the installation is invalid.
2025-02-14T18:37:34Z app[17815ee1c522d8] lhr [info]Execute the following command to install the binary used to process JS:
2025-02-14T18:37:34Z app[17815ee1c522d8] lhr [info]    mix esbuild.install
2025-02-14T18:37:34Z app[17815ee1c522d8] lhr [info]    (beacon 0.4.0) lib/beacon/runtime_js.ex:188: Beacon.RuntimeJS.validate_esbuild_install!/0
2025-02-14T18:37:34Z app[17815ee1c522d8] lhr [info]    (beacon 0.4.0) lib/beacon/runtime_js.ex:13: Beacon.RuntimeJS.build/1
2025-02-14T18:37:34Z app[17815ee1c522d8] lhr [info]    (beacon 0.4.0) lib/beacon/runtime_js.ex:128: Beacon.RuntimeJS.load!/1
2025-02-14T18:37:34Z app[17815ee1c522d8] lhr [info]    (beacon 0.4.0) lib/beacon/loader/worker.ex:397: Beacon.Loader.Worker.handle_call/3
2025-02-14T18:37:34Z app[17815ee1c522d8] lhr [info]    (stdlib 6.2) gen_server.erl:2381: :gen_server.try_handle_call/4
2025-02-14T18:37:34Z app[17815ee1c522d8] lhr [info]    (stdlib 6.2) gen_server.erl:2410: :gen_server.handle_msg/6
2025-02-14T18:37:34Z app[17815ee1c522d8] lhr [info]    (stdlib 6.2) proc_lib.erl:329: :proc_lib.init_p_do_apply/3

We mention that the step should be after RUN mix release but it should also clarify that it should also be after RUN mix assets.deploy