Add check for correct certbot image (amd64/arm32/arm64)

[jogerj]

Adds a check for current architecture so docker grabs the correct images accordingly. Since certbot only provides amd64, arm32 and arm64 docker images, only those checks are made. This will change certbot image in docker-compose.yml automatically:

• x86_64 (amd64): certbot/certbot:latest
• arm, armv7, etc. : certbot/certbot:armv6-latest
• aarch64, aarch64_be, armv8b, armv8l, etc.: certbot/certbot:armv8-latest

Resolves #109 and #87