[Mail] Use full chain instead of single cert for mail server

See https://community.letsencrypt.org/t/cannot-verify-domain-with-openssl/11545

pica-mail/pica-mail-mda/Dockerfile

@@ -22,7 +22,7 @@ ENV USER_FILTER (uid=%n)
 ENV PASSWORD_FILTER (uid=%n)
 # SSL
 # L'antislash "\" est nécessaire devant le slash "/" !
-ENV SSL_CERT "\/certs-ssl\/cert.pem"
+ENV SSL_CERT "\/certs-ssl\/fullchain.pem"
 ENV SSL_KEY "\/certs-ssl\/privkey.pem"
 
 COPY fichiers_de_configuration.sh /

pica-mail/pica-mail-mta/Dockerfile

@@ -47,7 +47,7 @@ ENV LMTP_PORT 24
 ENV SMTPD_CLIENT_RESTRICTIONS reject_rbl_client sbl.spamhaus.org, reject_rbl_client dnsbl.sorbs.net
 
 # SSL
-ENV SSL_CERT "/certs-ssl/cert.pem"
+ENV SSL_CERT "/certs-ssl/fullchain.pem"
 ENV SSL_KEY "/certs-ssl/privkey.pem"
 
 #configuration de OpenDKIM