install the exact version

or approximate, because apk wants the -rcX too

.woodpecker.yml

@@ -8,6 +8,7 @@ pipeline:
     - latest
     build_args:
     - RUBY_VERSION=${RUBY_VERSION}
+    - RUBY_PATCH=${RUBY_PATCH}
     - ALPINE_VERSION=${ALPINE_VERSION}
     - BASE_IMAGE=registry.nulo.in/sutty/monit
     username: sutty

Dockerfile

@@ -1,13 +1,12 @@
 ARG ALPINE_VERSION=3.13.6
-ARG RUBY_VERSION=2.7
-ARG RUBY_PATCH=5
 ARG BASE_IMAGE=alpine
 ARG EMAIL=monit@sutty.nl
 FROM ${BASE_IMAGE}:${ALPINE_VERSION} as build
+ARG RUBY_VERSION=2.7
+ARG RUBY_PATCH=5
 ENV EMAIL ${EMAIL:-monit@sutty.nl}
 
-RUN apk add --no-cache ruby patch
-RUN test "${RUBY_VERSION}.${RUBY_PATCH}" = `ruby -e 'puts RUBY_VERSION'`
+RUN apk add --no-cache ruby~${RUBY_VERSION}.${RUBY_PATCH} patch
 
 # Patch rubygems to install x86_64-linux-musl gems
 COPY ./rubygems-platform-musl.patch /tmp/
@@ -15,11 +14,11 @@ RUN cd /usr/lib/ruby/${RUBY_VERSION}.0 && patch -Np 0 -i /tmp/rubygems-platform-
 
 FROM ${BASE_IMAGE}:${ALPINE_VERSION}
 ARG ALPINE_VERSION
+MAINTAINER "f <f@sutty.nl>"
 ARG RUBY_VERSION
 ARG RUBY_PATCH
-MAINTAINER "f <f@sutty.nl>"
 
 # Install ruby and extracted gems and copy patched rubygems
-RUN apk add --no-cache tzdata ruby && rm /usr/lib/ruby/${RUBY_VERSION}.0/rubygems.rb
+RUN apk add --no-cache tzdata ruby~${RUBY_VERSION}.${RUBY_PATCH} && rm /usr/lib/ruby/${RUBY_VERSION}.0/rubygems.rb
 RUN apk add --no-cache ruby-json ruby-bigdecimal ruby-rake ruby-bundler ruby-irb ruby-io-console ruby-etc
 COPY --from=build /usr/lib/ruby/${RUBY_VERSION}.0/rubygems.rb /usr/lib/ruby/${RUBY_VERSION}.0/