[Mail] Fix SSL environment variables

pica-mail/pica-mail-mta/Dockerfile

@@ -47,9 +47,8 @@ ENV LMTP_PORT 24
 ENV SMTPD_CLIENT_RESTRICTIONS reject_rbl_client sbl.spamhaus.org, reject_rbl_client dnsbl.sorbs.net
 
 # SSL
-# L'antislash "\" est nécessaire devant le slash "/" !
-ENV SSL_CERT "\/certs-ssl\/cert.pem"
-ENV SSL_KEY "\/certs-ssl\/privkey.pem"
+ENV SSL_CERT "/certs-ssl/cert.pem"
+ENV SSL_KEY "/certs-ssl/privkey.pem"
 
 #configuration de OpenDKIM
 COPY spam/opendkim.conf /etc/