Fix dependency cert error on container build

[MaxAlex]

The container build process is currently failing in CI and on the merge action, with an error related to SSL certificates:

#13 [builder  2/16] RUN mix local.hex --force &&     mix local.rebar --force
#13 0.874 ** (Mix) httpc request failed with: {:failed_connect, [{:to_address, {~c"builds.hex.pm", 443}}, {:inet, [:inet], {:tls_alert, {:unsupported_certificate, ~c"TLS client: In state wait_cert_cr at ssl_handshake.erl:2199 generated CLIENT ALERT: Fatal - Unsupported Certificate\n {key_usage_mismatch,{{'Extension',{2,5,29,15},true,[keyCertSign,cRLSign]},\n                      {'Extension',{2,5,29,37},false,[{1,3,6,1,5,5,7,3,1}]}}}"}}}]}
#13 0.874 
#13 0.874 Could not install Hex because Mix could not download metadata at https://builds.hex.pm/installs/hex-1.x.csv.
#13 0.874 
#13 0.874 Alternatively, you can compile and install Hex directly with this command:
#13 0.874 
#13 0.874     $ mix archive.install github hexpm/hex branch latest
#13 0.874 
#13 ERROR: process "/bin/sh -c mix local.hex --force &&     mix local.rebar --force" did not complete successfully: exit code: 1

Assuming data_platform rebuilds the container on deploy, the service would currently be unable to come up if we deployed any changes, which blocks the earlier PR for EC_STAGE.METRIC_HISTORY.

It appears that the certificate error is preventing docker from installing hex and rebar3 via mix; hex is available via github and rebar3 is available on apk, so this PR uses those to install instead. Neither seems to be version-pinned coming from mix.

---

• To see the specific tasks where the Asana app for GitHub is being used, see below:
  • https://app.asana.com/0/0/1216636342478481