Update base image

[matejb]

To fix security variabilities.

✗ High severity vulnerability found in openssl/libcrypto1.1
  Description: NULL Pointer Dereference
  Info: https://snyk.io/vuln/SNYK-ALPINE39-OPENSSL-588029
  Introduced through: openssl/[email protected], openssl/[email protected], apk-tools/[email protected], libtls-standalone/[email protected], ca-certificates/ca-certificates@20191127-r2, curl/[email protected], libssh2/[email protected]
  From: openssl/[email protected]
  From: openssl/[email protected] > openssl/[email protected]
  From: apk-tools/[email protected] > openssl/[email protected]
  and 8 more...
  Introduced in your Dockerfile by 'RUN apk add ca-certificates curl'
  Fixed in: 1.1.1g-r0

✗ High severity vulnerability found in musl/musl
  Description: Out-of-bounds Write
  Info: https://snyk.io/vuln/SNYK-ALPINE39-MUSL-458529
  Introduced through: musl/[email protected], pax-utils/[email protected], alpine-baselayout/[email protected], openssl/[email protected], openssl/[email protected], zlib/[email protected], apk-tools/[email protected], libtls-standalone/[email protected], busybox/[email protected], ca-certificates/ca-certificates@20191127-r2, curl/[email protected], curl/[email protected], libssh2/[email protected], nghttp2/[email protected], musl/[email protected], busybox/[email protected], libc-dev/[email protected]
  From: musl/[email protected]
  From: pax-utils/[email protected] > musl/[email protected]
  From: alpine-baselayout/[email protected] > musl/[email protected]
  and 15 more...
  Introduced in your Dockerfile by 'RUN apk add ca-certificates curl'
  Fixed in: 1.1.20-r5