elixir:1.7.4-alpine breaking change

[Tom Crossland]

The docker image for elixir:1.7.4-alpine has changed recently (5 hours ago) and causes compatibility issues due to changes in the base images (erlang:21-alpine, alpine:3.9).

[load_failed,"Failed to load NIF library: 'Error loading shared library libcrypto.so.1.1: No such file or directory (needed by /app/lib/crypto-4.4.1/priv/lib/crypto.so)'","OpenSSL might not be installed on this system.\n"]

[Tom Crossland]

The issue was occurring in a multi-stage build when the runtime image was using alpine:3.8, solved by updating runtime image to use alpine:3.9.

See also #104, #105.