chore: Simplify yarn and alpine version update

Before commit 3f6cb08 (fix: only update yarn if there are other changes
to the dockerfile), yarnVersion depended on SKIP, so the substitute that
followed made sense. This commit removed the condition and changed it to
just substitute the version with itself, which doesn't make much sense.

Just remove it, and also move the real replace inside SKIP condition.

Also postpone retrieving the version to where it is used, and remove the
initial assignment to alpine_version, which is never used.

Author: orgads

update.sh

@@ -65,11 +65,6 @@ fi
 # TODO: Should be able to specify target architecture manually
 arch=$(get_arch)
 
-if [ "${SKIP}" != true ]; then
-  alpine_version=$(get_config "./" "alpine_version")
-  yarnVersion="$(curl -sSL --compressed https://yarnpkg.com/latest-version)"
-fi
-
 function in_versions_to_update() {
   local version=$1
 
@@ -135,9 +130,6 @@ function update_node_version() {
     sed -Ei -e 's/^FROM (.*)/FROM '"$fromprefix"'\1/' "${dockerfile}-tmp"
     sed -Ei -e 's/^(ENV NODE_VERSION ).*/\1'"${nodeVersion}"'/' "${dockerfile}-tmp"
 
-    currentYarnVersion="$(grep "ENV YARN_VERSION" "${dockerfile}" | cut -d' ' -f3)"
-    sed -Ei -e 's/^(ENV YARN_VERSION ).*/\1'"${currentYarnVersion}"'/' "${dockerfile}-tmp"
-
     # shellcheck disable=SC1004
     new_line=' \\\
 '
@@ -172,11 +164,10 @@ function update_node_version() {
     if diff -q "${dockerfile}-tmp" "${dockerfile}" > /dev/null; then
       echo "${dockerfile} is already up to date!"
     else
-      if [ "${SKIP}" = true ]; then
-        # Get the currently used Yarn version
-        yarnVersion="$(grep "ENV YARN_VERSION" "${dockerfile}" | cut -d' ' -f3)"
+      if [ "${SKIP}" != true ]; then
+        yarnVersion="$(curl -sSL --compressed https://yarnpkg.com/latest-version)"
+        sed -Ei -e 's/^(ENV YARN_VERSION ).*/\1'"${yarnVersion}"'/' "${dockerfile}-tmp"
       fi
-      sed -Ei -e 's/^(ENV YARN_VERSION ).*/\1'"${yarnVersion}"'/' "${dockerfile}-tmp"
       echo "${dockerfile} updated!"
     fi