I found I can not send email with php in dokcer, because the ssmtp not installed.

I added following section in Dockerfile:

...
# install MTA
RUN apt-get update && apt-get install -y ssmtp

MAINTAINER hongyi.chen

# Sendmail
COPY ./ssmtp.conf /etc/ssmtp/ssmtp.conf
...

The SSMTP config like this:

root=postmaster
AuthUser={{smtp_username}}
AuthPass={{smtp_password}}
mailhub={{smtp_host}}:{{smtp_port}}
UseTLS=YES
UseSTARTTLS=YES
rewriteDomain={{force_domain}}
hostname={{host_name}}

Ref:

https://linuxhandbook.com/linux-send-email-ssmtp/

https://www.techrepublic.com/blog/it-security/use-ssmtp-to-send-e-mail-simply-and-securely/