diff --git a/Dockerfile b/Dockerfile
index cc1a3d6..70e2df8 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,10 +1,14 @@
-FROM python:3.9.15-buster
+FROM python:3.9.15-slim-buster
 
 RUN apt-get update
 RUN apt-get install -y \
-	android-tools-adb \
-	android-tools-fastboot
-RUN pip3 install protobuf
+    android-tools-adb \
+    android-tools-fastboot \
+    unzip \
+    curl \
+    git
+RUN pip3 install protobuf \
+    six
 COPY . /magisk
 WORKDIR /magisk
 CMD bash reinstall-magisk-on-lineageos
diff --git a/reinstall-magisk-on-lineageos b/reinstall-magisk-on-lineageos
index 085fc40..ee3104a 100755
--- a/reinstall-magisk-on-lineageos
+++ b/reinstall-magisk-on-lineageos
@@ -2,15 +2,19 @@
 
 set -e
 
+print_message() {
+    printf "$1\n"
+}
+
 PHONE_ADB_SERIAL_NUMBER=${ADB_DEVICE} # The adb serial number can be found by running `adb devices`.
 if [$PHONE_ADB_SERIAL_NUMBER = '']; then
     PHONE_ADB_SERIAL_NUMBER=$(echo $(adb devices) | sed -rn 's/(^|(.* ))([^ ]*) device(( .*)|$)/\3/; T; p; q')
+    if [[ -z $PHONE_ADB_SERIAL_NUMBER ]]; then
+        print_message "ERROR: no phone connected or connection unauthorized"
+        exit 1
+    fi
 fi
 
-print_message() {
-    printf "$1\n"
-}
-
 check_phone_is_connected() {
     if ! adb devices | grep --silent "$PHONE_ADB_SERIAL_NUMBER"; then
         print_message "ERROR: phone '$PHONE_ADB_SERIAL_NUMBER' is not connected"
@@ -38,7 +42,7 @@ get_device_downloads_page() {
 }
 
 get_build_url() {
-    curl --no-progress-meter "$(get_device_downloads_page)" |
+    curl --silent "$(get_device_downloads_page)" |
         grep --only-matching --ignore-case \
             "https://mirrorbits.lineageos.org/[^\"]*"$(
             )"$(get_lineage_version)-signed.zip" |
@@ -46,12 +50,15 @@ get_build_url() {
 }
 
 check_exists_latest_lineageos_build() {
-    # The --location lets curl follow the redirection.
-    curl --location "$(get_build_url)?sha256" --output /tmp/lineageos.sha256
-    hash1=$( awk '{print $1}' lineageos.sha256 )
-    hash2=$( sha256sum lineageos.zip | awk '{print $1}' )
-    [[ "$hash1" =~ "$hash2" ]]
-    return
+    if [[ -f "/tmp/lineageos.zip" ]]; then
+        # The --location lets curl follow the redirection.
+        curl --silent --location "$(get_build_url)?sha256" --output /tmp/lineageos.sha256
+        hash1=$( awk '{print $1}' /tmp/lineageos.sha256 )
+        hash2=$( sha256sum /tmp/lineageos.zip | awk '{print $1}' )
+        [[ -n $hash1 && -n $hash2 && $hash1 == $hash2 ]]
+        return
+    fi
+    false
 }
 
 download_latest_lineageos_build() {
@@ -151,15 +158,14 @@ check_magisk_app() {
     magisk_package_name_count=$(adb -s "$PHONE_ADB_SERIAL_NUMBER" shell pm list packages "$magisk_package_name" | wc -l)
     if [ "${magisk_package_name_count}" -eq 1 ]
     then
-        print_message " [OK] Magisk app seems to be installed on selected phone"
+        print_message "Magisk app seems to be installed on selected phone"
     elif [ "${magisk_package_name_count}" -eq 0 ]
     then
-        print_message " [ERROR] Magisk app seems not installed on selected phone. Exiting."
+        print_message "ERROR: Magisk app seems not installed on selected phone."
         exit -1
     else
-        print_message " [ERROR] More than one Magisk app entry is present:"
+        print_message "ERROR: More than one Magisk app entry is present:"
         adb -s "$PHONE_ADB_SERIAL_NUMBER" shell pm list packages "$magisk_package_name"
-        print_message "Exiting."
         exit -1
     fi
 }
@@ -175,10 +181,11 @@ main() {
     is_rooted_debugging_enabled
     print_message "Checking on phone if Magisk is installed"
     check_magisk_app
+    print_message "Checking if build archive already exists"
     if check_exists_latest_lineageos_build; then
-        print_message "Build archive already exists, skipping download"
+        print_message "Build archive found, skipping download"
     else
-        print_message "Downloading build archive from $(get_build_url)"
+        print_message "Build archive not found or hash mismatch, downloading from $(get_build_url)"
         download_latest_lineageos_build
     fi
     print_message "Extracting 'boot.img' from build archive in /tmp/boot.img"