Fix docker build environment

[ehaupt]

This fixes:
#1739

[yoyama]

@ehaupt Thank you for your contribution.

I tested your PR but I found another failure on docker-build.

#8 97.24 Hit:5 http://security.ubuntu.com/ubuntu bionic-security InRelease
#8 97.27 Hit:6 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
#8 97.43 Err:7 https://apt.postgresql.org/pub/repos/apt bionic-pgdg Release
#8 97.43   Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 72.32.157.246 443]
#8 97.43 Hit:8 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
#8 97.64 Reading package lists...
#8 98.80 E: The repository 'https://apt.postgresql.org/pub/repos/apt bionic-pgdg Release' does not have a Release file.

So I added another fix to this PR.
https://github.com/treasure-data/digdag/pull/1740/files#diff-8c9324bfc3666c3f067d0c41e00233f89474c536b0db7491f961f4eac9a0a882R5
How about your thought ?
If you are ok, I will approve it.

[ehaupt]

At the time this issue was submitted the additional ca-bundle was not required. Looks like this has changed.

If you are ok, I will approve it.

Definitely. Thank you.

[yoyama]

LGTM